Reduction

The organism makes use of reduction in some cases where t h e oxidative enzymes fail. The conversion of chloralhydrate and butylchloralhydrate into t h e corresponding alcohols are examples. Many ketones, especially cyclic ketones, are reduced, for instance, progesterone into pregnanediol (105). For some molecular configurations, reduction seems to be t h e only possible cata-bolic route, viz., for t h e azo configuration, e.g., t h e conversion of prontosil into sulfanilamide, and for t h e nitro groups, e.g., t h e conversion of nitrophenols into aminophenols.

Many esters foreign t o t h e body are hydrolyzed by t h e enzymes of blood plasma and cells. Where serum esterases are concerned, t h e corresponding amides are frequently more stable t h a n t h e esters. Procainamide is more stable t h a n procaine, for instance. Substitutions on t h e α-carbon atom of t h e alcohol and/or the acid portion of t h e ester, tend t o stabilize it against hydrolysis by plasma esterases (123). This is again an example of packing of t h e essential group (see Fig. 1). Here, too, a steric hindrance with respect t o t h e active site on t h e esterase is feasible.

The resistance of lidocaine (Xylocaine) and 2,6-dimethoxybenzoylcholine against enzymatic hydrolysis is a further example of t h e influence of packing of t h e vital points in drugs on their stability (124, 194).

Thomas and Stoker (194) studied t h e enzymatic (acetylcholinesterase) and nonenzymatic (OH ions) hydrolysis of various or^o-substituted benzoyl-cholines and t h e inhibition of t h e hydrolysis of acetylcholine by acetylcholin­

es.*.3. Hydrolysis

Ι , Ι . Β . DRUG T R A N S F E R E N C E ; DRUG METABOLISM 59 esterase b y means of these benzoylcholines. This s t u d y gives a good basis for t h e discussion of some factors t h a t play a role in t h e relation between t h e chemical structure a n d t h e r a t e of hydrolysis. The change in t h e nonenzymatic hydrolysis as a result of t h e various substitutions in benzoylcholine is due t o a change in t h e stability of t h e ester bond as such. I t m a y result from inductive effects a n d other effects leading t o a change in t h e intramolecular distribution of charges. The enzymatic hydrolysis is also dependent on t h e ester bond stability. Here a steric hindrance on t h e enzyme surface, especially with respect to t h e approach between t h e vital spot (the ester group in t h e drug) a n d its

rate of hydrolysis in human serum

R2 y / m l / h r

Η Η 500

Η CH3 15

CH3 CH3 0

FIG. 1. Influence of "packing" of the ester group on the stability of drugs.

F r o m Table IV.A it m a y be seen t h a t t h e nonenzymatic hydrolysis, t h e en­

zymatic hydrolysis, and t h e affinity of t h e benzoylcholines t o t h e enzyme acetylcholinesterase, as represented by anti-acetylcholinesterase activity, are not strictly correlated. This means t h a t each of t h e three processes depends on t h e structure in its own way. The decrease in t h e r a t e of hydrolysis does not necessarily lead t o a decrease in cholinergic a c t i v i t y ; this m a y even increase (194a, 194b).

Possibly also t h e resistance of certain penicillins against penicillinase is based on such factors as mentioned for t h e benzoylcholines. 2,6-Dime-thoxyphenylpenicillin is not only penicillinase-resistant b u t it is also found t o act as a competitive inhibitor of t h e B. cereus penicillinase (161). The bond in penicillin split by penicillinase is t h e lactam bond which, although situated closely t o t h e 2,6-substituted ring, is not directly adjacent t o it as in t h e case of t h e benzoylcholines mentioned. The bis-or^o-substituted derivative, 5-methyl-3-phenyl-4-isoxazolyl-penicillin is also a competitive inhibitor of

60 Ε. J . A R I E N S AND A. M. SIMONIS

penicillinase. Experiments of Citri etal. (48a, b, 75b) m a k e it probable t h a t as a result of the or^o-substitution in t h e side chain of penicillin t h e conformation of t h e active site on t h e penicillinase is changed in such a way t h a t t h e enzymatic hydrolysis cannot take place. I n general t h e character of t h e side-chain in t h e various penicillins, especially α-substitution, clearly influences the rate of hydrolysis by penicillinase (10a, 48a, 54a, 75b).

T A B L E I V . A

NONENZYMATIC HYDROLYSIS, ENZYMATIC HYDROLYSIS ( A C H ESTERASE), AND ANTI-A C H ESTERANTI-ASE ANTI-ACTIVITY IN VANTI-ARIOUS Ortho-SUBSTITUTED BENZOYLCHOLINE COMPOUNDS'*

^ T J- O - O - C - C .

?

⁺

—Ν—C 1 c

Nonenzymatic

hydrolysis⁶ Enzymatic hydrolysis⁶

Anti-ACh esterase activity^c

2-H 1.00 1.00 1.00

2-CH3 0.65 0.24 3.10

2-C1 1.40 1.30 8.81

2-Br 1.02 0.70 4.20

2-J 0.74 0.24 14.50

2 - N 02 1.40 0.19 8.58

2-OCH3 0.77 0.73 6.64

2,6-diCl 0 0 6.26

2,6-diCH3 0 0 —

2,4,6-triBr 0 0 5.80

2,4,6-triN02 3.40 0 8.23

° The ratio enzymatic: nonenzymatic hydrolysis for benzoylcholine is 5.30.

After Thomas et al. (194)

b Relative rate; benzoylcholine = 1.00.

c Relative activity tested on horse serum cholinesterase; benzoylcholine = 1.00.

Interesting studies on t h e relations between t h e chemical structure of amides and t h e rate of hydrolysis by amidases from r a b b i t liver were performed by B r a y et al. (27). Some of their results are summarized in Table I V . B .

The decrease in t h e velocity of hydrolysis for compounds with higher values for n, m a y be ascribed t o a decrease in t h e concentration of free molecules.

This results from t h e increasing tendency of these compounds t o form micelles (91).

I n addition to esterases and amidases, glucosidases also t a k e p a r t in enzym­

atic hydrolysis.

I.B.1.4. Conjugation

I n conjugation, drug molecules are bound t o other molecules by elimination of water a n d formation of esters, amides, and other substitution products (26).

Ι , Ι . Β . DRUG T R A N S F E R E N C E : DRUG METABOLISM 61

C H3( C H2)NC O N H2 / \ _ ( C H2)N— C O N H2

η (%) (%)

0 2 4 4

1 3 1 7

2 1 3 1 0 5

3 5 5 5 8

4 9 0 2 6

5 9 0

6 6 3

7 3 8

8 1 8

9 8

1 0 8

1 1 3

1 2 8

1 3 2

1 4 3

a From Bray (27).

One or both reacting molecules t a k e p a r t in a n activated state. Usually, t h e active molecule is derived from metabolic processes of t h e organism. Well-known examples are *' active " acetate, sulfate, a n d glucuronic acid. Sometimes t h e drug or its metabolites, e.g., benzoic acid a n d phenylacetic acid, are acti­

vated, t h a t is, bound t o coenzyme A (CoA). The active products are usually formed b y mitochondrial intervention in t h e liver cells. A transferase accom­

plishes t h e transfer of t h e activated molecule t o t h e other members of t h e conjugation pair.

These transferases sometimes demonstrate a certain degree of specificity with respect t o t h e drug a n d are also located in t h e mitochondria of t h e liver ceUs (35, 60, 121a,b).

1. Conjugation with glucuronic acid occurs with alcohols resisting oxidation (demonstrated in Table I) a n d with phenolic OH-groups. Carboxyl groups, especially if t h e y are located on a n aromatic nucleus or close t o it, are in some cases converted t o glucuronides, e.g., benzoic acid, phenylacetic acid a n d diphenylacetic acid. T h e same holds t r u e for carboxyl groups with a heavy a-carbon-substitution. Take, for instance, trimethylacetic acid a n d t e r t i a r y butylacetic acid (61). " A c t i v e " glucuronic acid, uridine diphosphoglucuronic acid, is t h e source of glucuronic acid (Fig. 2) (60, 87). Some glucuronic acid

T A B L E I V . Β

PERCENTAGE OF AMIDES HYDROLYZED IN 5 H R AT P H 7 . 4 BY RABBIT-LIVER EXTRACT"

62 Ε. J . A R I E N S AND A. M. SIMONIS

transferases in liver microsomes effect t h e transfer under formation of ether-, ester-, and other glucuronides, depending on t h e substrate (60, 60a, 214).

2. Like glucuronic acid, amino acids are conjugated with acids. The foreign

COOH n N

Κ ° ° V Y

(oh Y — O — P — O — P — O — C ^ ^ O ^ ^ N ^ ^ HtT f OH OH

OH

HO OH uridine diphosphoglucuronic acid

OH CI

+ H O - C - C — C I I I

ci

trichloroethanol

COOH

Μ i¹ jfoH ) 2- 0 - C — c— CI + uridine-diphosphate

HO ( CI OH

trichloroethyl glucuronide

FIG. 2. Glucuronic acid conjugation of trichloroethanol.

acids, activated as acyl-CoA, react with t h e amino groups of an amino acid.

Mammals usually conjugate with glycine, b u t m a n a n d ape sometimes with glutamine. Examples are t h e formation of hippuric acid, phenaceturic acid,

Ο

// \\

C O O H + A T P + HS — R - A M P + pyrophosphate benzoic acid coenzyme A benzoyl-coenzyme A

Ο , V Ο

C - S — R + H?N — C — C O O H γ \ ) C~ N H — C — C O O H + H S — R

benzoyl-coenzyme A glycine hippuric acid coenzyme A FIG. 3. Glycocol conjugation of benzoic acid.

nicotinuric acid from t h e corresponding acids, benzoic acid, phenylacetic acid, and nicotinic acid (Fig. 3). Birds use ornithine instead of glycine; t h e y convert benzoic acid to ornithuric acid, which is t h e analog of hippuric acid. Certain spiders use glutamic acid and arginine for conjugation of aromatic acids (166a).

Ι,Ι.Β. DRUG TRANSFERENCE: DRUG METABOLISM 63

Ο Ο II II HO—S—Ο—Ρ—Ο—C

Ο OH I I

Ο OH HO—l>-OH

ο

II

adeno sine - 3' - phosphate - 5' - phospho sulfate

HO—S— Ο - γ y + a d e n o s i n e - 3 ' , 5'-diphosphate Ο

\=s

phenol phenolsulfate

FIG. 4. Sulfate conjugation of phenol.

CH3

u

ooc—c—c - C —

NH2

S —

COOH

HO OH

S-adenosyl methionine nicotinic acid

COOH

+ adenosine + homocysteine

trigonelline

FIG. 5. Methylation of nicotinic acid.

64 Ε . J . ARIENS AND Α. Μ. SIMONIS

3. Acetylation is another t y p e of conjugation. Foreign compounds bearing amino groups not suitable for oxidative deamination, mainly t h e aromatic amines, e.g., t h e anilides, are conjugated with acetic acid ( " a c t i v e " acetate) derived from acetyl-CoA. The acetylation of various sulfonamides m a y serve as an example (120a).

4. Phenolic hydroxyl groups m a y be conjugated with glucuronic acid b u t also with sulfuric acid, which results in t h e formation of ethereal sulfates.

" A c t i v e " sulfate, 3'-phosphoadenosine-5'-phosphosulfate serves as a source for sulfate (Fig. 4) (23,82b). A sulfuric acid transferase brings about t h e transfer to t h e foreign compound. The formation of paracresylsulfuric acid from para-cresol is an example.

/ \

iOOH Ο iodobenzol c y s t e i n e acetyl c o e n z y m e A

I ft \ \ — S — C — C — N H — C — C H3 + R—SH COOH Ο

/>-iodophenylmercapturic acid c o e n z y m e A FIG. 6. Cysteine conjugation of iodobenzol.

5. Methylation is another t y p e of bio-transformation. I t takes place with nitrogen in aromatic heterocyclic rings, for instance, t h e formation of iV¹ -methylnicotinamide from nicotinamide (165). Phenolic OH-groups in t h e catecholamines, e.g., epinephrine (13,14, 82), and in steroids, e.g., estrone a n d estradiol, m a y be methylated t o m e t h o x y compounds (71). #-Adenosylmeth-ionine probably serves as a source for t h e methyl groups in this case (Fig. 5) (12, 16a).

6. Finally, mercapturic acid synthesis m a y be mentioned, whereby cysteine is substituted for hydrogen on aromatic rings, or for halogen atoms on aromatic rings or aliphatic chains (Fig. 6). This is of importance in t h e dehalogenation of foreign compounds, for example, halogenobenzene derivatives (18, 26b, 30).

I.B.1.5. Chemical Antagonism or Antagonism by Neutralization This t y p e of antagonism has something in common with conjugation. Here, too, t h e inactivation of a drug takes place as a result of its coupling with other molecules. This time t h e molecules are not of endogenous origin, as were glucuronic acid a n d acetic acid, nor are enzymes concerned with their binding

Ι,Ι.Β. DRUG TRANSFERENCE: DRUG METABOLISM 65 t o t h e drug. B o t h reacting compounds are drugs; one as a rule is a toxin, t h e other an antidote. The t y p e of reaction is often chelation. I n t h e paragraph concerned with chelation as a mechanism of t r a n s p o r t (Section I.A.1.3.C), various examples of this t y p e of bio-inactivation are mentioned: Ca++<-» cit­

r a t e , Hg++<->BAL,* Pb++<->EDTA,f Cu++<->penicillamine. Some more ex­

amples, curare<-> congo red, a n d curare<->germanin, will be mentioned in t h e following chapters. A q u a n t i t a t i v e theory on this t y p e of bio-inactivation is proposed by Gaddum (75) a n d Ariens (9). Because of t h e similarity of t h e dose-response curves for t h e competitive antagonism a n d for t h e chemical antagon­

ism, it will get special a t t e n t i o n in Section I I . B . 3 . I.B.1.6. Multiple Metabolic Pathways

Compounds foreign t o t h e body m a y be catabolized simultaneously along different roads.

1. One chemical group in t h e drug m a y be processed in different ways. Take, for instance, phenol. I t is p a r t l y converted t o its glucuronide a n d t o its sulfuric acid ether. Benzoic acid given orally t o h u m a n s , dogs, or pigs m a y be recovered from t h e urine p a r t l y conjugated with glycine t o hippuric acid a n d p a r t l y as t h e glucuronide (212).

2. A drug molecule m a y be a t t a c k e d a t different points. An example is

^-aminosalicylic acid. This compound contains 3 groups suitable for conjuga­

tion ; an amino group bound t o t h e ring; a carboxyl group bound t o t h e ring;

a n d a phenolic OH-group. After oral administration, a large fraction (42%) is excreted conjugated a t t h e carboxyl group with glycine, 2 9 % is found in t h e urine as an iV^-acetylated derivative, 2 5 % is excreted unchanged, while little if a n y of the drug is conjugated a t t h e phenolic OH-group (120).

N o t only conjugations b u t also different types of metabolic processes m a y t a k e place simultaneously. Salicylic acid is excreted in t h e urine, mainly conjugated with glycine a t t h e carboxyl group, as salicyluric acid ( 5 0 - 6 0 % ) ; a fraction is conjugated with glucuronic acid (20-25%). Two types are found, one probably conjugated in t h e carboxyl group, t h e other in t h e phenolic OH-group. A small fraction appears as oxidation products. One of these is gentisic acid (2,5-dihydroxybenzoic acid) (5, 84, 181). Also, for morphine and related compounds, a variety of metabolic changes occur in t h e body (121). Blockade of one metabolic p a t h w a y can lead to an increased metabolism of t h e drug along other routes (165a).

For more detailed and extensive information on drug metabolism t h e reader is referred t o William's well-known monograph (212, 214) a n d t o various review articles (15, 26a, 34, 35, 68, 68a, 78b, 121,122, 138a,b, 164,189, 208). A valuable discussion of t h e kinetics of metabolism of foreign organic compounds is given by Teorell (193) a n d by B r a y (28).

* British Anti-Lewisite.

t Ethylenediaminetetraacetic acid.

66 Ε . J . A R I E N S AND Α. Μ. SIMONIS

I.B.2. FACTORS I N F L U E N C I N G D R U G METABOLISM The relation between t h e dose of a drug and t h e effective concentration of t h e active drug is t h e resultant of a n u m b e r of detail processes. The various catabolic routes for a drug can be considered as parallel flow systems. The drug will be divided among these systems according to t h e capacity of each of t h e m . The relative capacity of t h e various catabolic systems for a drug, a n d therefore t h e fraction of t h e drug processed by each of t h e m , depends on various factors, as for instance: (a) t h e chemical properties of t h e d r u g ; (b) t h e dose; (c) the route of administration; (d) t h e diet a n d drugs given t o t h e a n i m a l ; (e) t h e animal's species; (/) its sex; a n d (g) its individual variations.

I.B.2.1. Chemical Properties

The way in which a certain group in a compound, e.g., a phenolic OH-group, is processed varies with t h e place of such a group in t h e molecule. Table V gives

T A B L E V

INFLUENCE OF RING SUBSTITUTIONS ON METABOLISM OF HYDROXYBENZALDEHYDE AS S E E N BY THE EXCRETION PRODUCTS IN URINE OF RABBITS"

% of dose* excreted as:

Ether-soluble Ester Ether Ethereal acid glucuronide glucuronide sulfate

2-Hydroxybenzaldehyde 75 18 9 3 3-Hydroxybenzaldehyde 75 9 9 7 4-Hydroxybenzaldehyde 67 4 16 9

* From Bray (28).

b Dose level 0.4 gm/kg.

examples of t h e influence of t h e substitution of OH-groups on t h e ortho, meta or para place on t h e t y p e of conjugation of hydroxybenzaldehyde in t h e r a b b i t (28). Table V I demonstrates t h e influence of ortho-, meta- a n d para-chloro-substitution in acetanilide on metabolism of t h e drug in r a t s (141).

Differences in charge distribution on the drug molecules often play a role in drug metabolism (95a). I t has been observed, for instance, t h a t t h e r a t e of acetylation of amines is related to the electronic charge on the amino nitrogen (if not protonated). The rate of deacetylation of iV^-acetyl amines is reported as correlating with the dipositivity (the positive charge on the nitrogen atom and on the carbonyl carbon atom) of the iV-acetyl bond (146c).

Differences in t h e biological activity of t h e stereoisomers of a compound strongly suggest t h e implication of drug-receptor interactions. Such a

stereo-Ι , stereo-Ι . Β . DRUG T R A N S F E R E N C E : DRUG METABOLISM 67 specificity m a y have its origin not only in t h e interaction of t h e drug with t h e receptors essential t o t h e effect studied, b u t in t h e processes concerned with t h e drug transference as well. Serum esterases only hydrolyze t h e L( + ) isomer of acetyl-jS-methylcholine (79). After a dose of racemic mepacrine, an optically active mepacrine appears in t h e urine (85). The antipodes in d^glutethimide are degraded in different ways (42b, 113m). Certain permeases, specific penetra­

tion systems found in bacteria, exhibit stereo-specificity (50). The same m a y be t r u e for t h e displacement of drugs from silent receptors (19).

T A B L E V I

INFLUENCE OF RING SUBSTITUTION ON METABOLISM OF CHLOROACETANILIDES AS SEEN IN THE EXCRETION PRODUCTS IN THE U R I N E OF RATS AFTER INJECTION OF 2-, 3-,

AND 4-CHLOROACETANILIDE ( C l^{8 e})^a

Products

-C—CH8

ο

II

2-chloroacetanilide 3-ehloroacetanilide 4-chloroacetanilide (%) (%) (%)

4-OH-Derivatives⁶ 45.10 57.32 —

2-OH-Derivatives* — — 61.96 6-OH-Derivatives* 8.53 26.46 — Deacetylation 28.28 8.26 2.52 Unchanged 3.18 1.34 4.63 Chloride ions 0.62 1.98 1.18

"From Newell (141).

b After acid hydrolysis of the glucuronides, ethereal sulfates, etc.

The relatively high, in vivo amine-oxidase-inhibiting activity of isopropyl-hydrazine with respect t o n a t u r a l amino acids, as compared with t h e u n n a t u r a l , is t h o u g h t t o be related t o specific t r a n s p o r t mechanisms n o t present in in vitro experiments. I n t h e latter, t h e differences in amine-oxidase-inhibiting activity toward n a t u r a l a n d u n n a t u r a l compounds are m u c h smaller (15).

The d-isomers of various amino-acid derivatives inhibit, in a competitive way, t h e hydrolysis of t h e Z-isomers b y a-chymotrypsine (208). The d-isomer of a-phenoxyethyl phenylpenicillin inhibits t h e hydrolysis of t h e Z-isomer by staphylococcal penicillinase (186). The breakdown of Z-histidine b y liver histi-dase is inhibited by d-histidine (65). There is a competition between t h e enan-tiomorphs of methionine during intestinal absorption (9p, 100).

I n t h e isomer which is not or only slowly metabolized, one of t h e groups concerned m a y have a wrong orientation with respect to t h e active site on t h e enzyme. A group essential for the reaction m a y point in t h e wrong direction, a

68 Ε. J . ARIENS AND Α. Μ. SIMONIS

group not essential for the reaction m a y cause a steric hindrance, etc. As mentioned before, a " p a c k i n g " around the vital group in a drug molecule like α-methyl substitution and or£/io-substitution often strongly influences drug metabolism (see Table I, Fig. 1, a n d Table IV.A). The cause of this m a y be steric hindrance; however, in oxidative processes the absence of α-Η atoms which possibly play an essential role in the process m a y be the cause as well (20).

Most catabolic changes of drugs, especially oxidations, reductions, and con­

jugations, t a k e place intracellularly. Many types of hydrolysis occur mainly in t h e plasma. Penetration of t h e cell and t h e functional units of t h e cell, microsomes and mitochondria, is essential for m a n y metabolic processes. This means t h a t a certain degree of lipid solubility will be an advantage t o these processes. Take, for instance, t h e compounds chlorothiazide and flumethiazide as compared t o t h e more lipophilic hydrogenated compounds, dihydrochloro-thiazide and dihydroflumedihydrochloro-thiazide. These lipophilic compounds are absorbed more completely from the g u t ; t h e volume of distribution is larger for dihydro-chlorothiazide t h a n for dihydro-chlorothiazide. The " d i h y d r o " compounds are reab­

sorbed in t h e tubules and, therefore, are excreted less rapidly in t h e urine.

Chlorothiazide is excreted in urine completely unchanged, while t h e " d i h y d r o "

compounds are excreted partly metabolized (132).

Many strong bases, e.g., a, ω-bis-trimethylammonium compounds (hexa-methonium, deca(hexa-methonium, etc.) a n d strong acids, e.g., sulfonic acids, ethereal sulfates, and phthalic acid are slowly absorbed from t h e gut, while t h e fraction reabsorbed in t h e tubules is small. These drugs are excreted mainly unchanged in t h e urine.

/.β.2.2. Effect of Dosage

The fraction of t h e dose of a drug subjected t o bio-transformation by a certain metabolic route m a y v a r y with t h e dose. I n h u m a n s when a dose of 400 mg andosterone is given orally, 4 . 4 % is recovered as t h e sulfate, a n d 4 8 % as t h e glucuronide. When a dose of 4000 mg is given, t h e recoveries are 2 1 % and 47 %, respectively (167). An increase in t h e doses of phenol given t o rabbits results in a decrease in the ratio sulfate conjugation .glucuronide conjugation.

This is probably because t h e sources of sulfate are restricted (29).

At a certain dosage the optimal capacity of a metabolic system m a y be reached. A further increase of t h e dosage will lead t h e n t o a switch t o other systems.

I t m a y be expected when combinations of drugs are given which are proc­

essed by t h e same metabolic system, t h a t a m u t u a l interference will occur on t h e basis of competition for active sites on enzymes, etc. p-Aminobenzoic acid is found t o compete with salicylic acid and with benzoic acid for t h e glycine conjugation system (181).

Drugs can compete for common metabolic systems. Probenecid reduces

Ι , Ι . Β . DRUG T R A N S F E R E N C E : DRUG METABOLISM 69 in vitro conjugation with glycine of p-aminobenzoic acid (PABA). The reduc­

tion in excretion of t h e PABA-glycine conjugate by probenecid m a y be p a r t l y due to t h e interference with t h e conjugation (20a).

I.B.2.3. Route of Application

This will be influential on drug metabolism for drugs which are rapidly metabolized in t h e liver, especially if oral a n d parenteral administration are compared. The liver plays an i m p o r t a n t role in drug metabolism. Examples of differences in drug metabolism dependent on t h e route of application have been described for ^-aminosalicylic acid (134), for cortisone a n d Prednisone (149), a n d for p a r a t h i o n a n d paraoxon (93). If drug metabolism in t h e liver leads t o a bio-inactivation, t h e parenteral application will be t h e most effective one. If bio-activation takes place in t h e liver, t h e oral route m a y be preferable (Section I.B.3.2).

Ι.Β.2Λ. Diet and Drugs

The fractions of benzoic acid excreted as hippuric acid, a n d as benzoylglu-curonide by t h e pig, vary with t h e supply of glycine in t h e diet. W i t h a protein-free diet, 6 0 % of t h e excreted benzoic acid is in t h e hippuric acid form; t h e addition of gelatin, rich in glycine, increases t h e hippuric acid fraction t o 8 9 % (53).

Animals on diets poor in sulfur-containing compounds, have difficulty in t h e bio-transformation of materials usually excreted as mercaptans. The ad­

dition of cysteine, cystine, or methionine leads t o an increased excretion of mercapturic acid conjugates. Sulfate conjugation of phenols given in high doses is increased by feeding t h e animals sulfate precursors like I-cystine a n d sodium sulfite. Glucuronide conjugation is t h e n decreased (29). A counterpart of these dietary requirements is t h e induction of a deficiency in cysteine as a result of t h e administration of high doses of compounds t h a t require cysteine for their conjugation. An instance of this process is demonstrated when bromo-benzene is given t o young animals on diets poor in t h e sulfur-containing amino acids (187,188).

Injection of glycine or glucuronic acid strongly decreases t h e toxicity of salicylic acid in dogs; t h e excretion of salicylglucuronic acid increases (78).

Administration of glucose, fructose, or glucuronolactone can accelerate glucuronide formation a n d excretion (18a, 94a).

Adaptive responses of t h e body t o drugs by increasing t h e metabolizing enzymes are common (114c, 134a). Such an a d a p t a t i o n is " crossed" for drugs processed along t h e same metabolic way. P r e t r e a t m e n t of r a t s with pheno-barbital increases t h e liver activity with respect to t h e metabolism of hexo-barbital and other drugs. A decrease in duration of anesthesia for barbiturates like hexobarbital is t h e result (50a, 113b, 115a, 118,158,158b). This is found to be due t o an increased inactivation of t h e drug.

70 Ε. J . ARIENS AND Α. Μ. SIMONIS

Changes in drug metabolism induced by p r e t r e a t m e n t with drugs can cause a tolerance for other drugs (113d, e,f, g,j, k, 114b, 158a, c). This will be the case if the metabolic process results in a bio-inactivation of these drugs. If, however, a bio-activation takes place, the p r e t r e a t m e n t m a y result in an increased sensitivity (see Sections LB.3.1 and 3.2.) As a m a t t e r of fact, application of drugs can lead to a change in metabolism of endogenous compounds, too, e.g., steroid hormones (76a; see also 21 and 156).

An adaptive increase of drug metabolism induced by drugs such as pheno­

barbetal and methylcholanthrene can be suppressed by inhibitors of protein synthesis like ethionine (50b, 113h).

Administration of various drugs stimulates t h e conversion of glucose t o glucuronic acid. This is often associated with increased excretion of ascorbic acid. This phenomenon is absent in t h e guinea pig, monkey, and m a n , species which lack t h e ability to convert glucuronic acid to ascorbic acid (42).

I n r a t s t h e increased excretions of ascorbic acid after drugs like Chloretone is attended with an increase in t h e concentration of lactonase (substrate D-glucuronolactone) in liver microsomes (113c).

Induction of enzyme synthesis by substrates is a common phenomenon in biochemistry. N o t only drugs, b u t also biological substrates are known to lead to adaptive increase in enzyme production. An interesting aspect is the specifi­

city of the induction (208a). I t appears t h a t the inducer does not necessarily have to be a substrate for the enzyme whose production is induced. For the induction of ^-glucuronidase, it is found t h a t bad substrates m a y be good

city of the induction (208a). I t appears t h a t the inducer does not necessarily have to be a substrate for the enzyme whose production is induced. For the induction of ^-glucuronidase, it is found t h a t bad substrates m a y be good

In document Section LB (Pldal 6-0)