C. The Role of Transport Processes across Mitochondrial Membranes in the Control of Lipogenesis
III. CONCLUDING COMMENTS
It is apparent that large numbers of inhibitors are available which block various stages of fatty acid oxidation and biosynthesis. For work on the properties of individual enzymes of these metabolic pathways, a host of effective agents has been enumerated. These inhibitors facilitate investigations on the kinetics and mechanisms of catalysis, but their use in more complex systems is less satisfying, for several reasons. The most problematic general difficulty is the lack of specificity of various inhibitors for single enzymic reactions when several multienzyme meta
bolic pathways are present. This, of course, is a problem common to the use of all inhibitors and can often be circumvented by judicious design of experiment. Difficulties less easily removed arise from the fact that the low rates of transport of inhibitors across various membranes prevent accumulation of adequate concentrations of inhibitor at the appropriate intracellular site. In studies on whole cells or organisms, transport rates are often limiting, thus necessitating the use of higher doses and thereby increasing the likelihood of nonspecific inhibition.
The availability of specific effective inhibitors of lipolysis, fatty acid activation, carnitine acyltransferase activity, anion transport across mitochondria, acetyl-CoA carboxylase activity, etc., in complex systems would permit evaluation of the physiological importance of each of these pathways in metabolic processes in the intact cell within the whole or
ganism. The importance of this evaluation may be considered in the context of a statement by Dubos, quoted by Dobzhansky (195). "In the most common and probably the most important phenomena of life, the constituent parts are so interdependent that they lose their character, their meaning and indeed their very existence, when dissected from the functioning whole. In order to deal with problems of organized com
plexity, it is therefore essential to investigate situations in which several interrelated systems function in an integrated manner."
The pathways of fatty acid metabolism provide many examples that support this generalization. We have briefly indicated that during times of carbohydrate deficiency, the rates of fatty acid oxidation are greatly increased in association with enhanced rates of lipolysis in adipose tissue and gluconeogenesis in liver (for reviews, see 6, 55). Simultaneously, fatty acid biosynthesis is depressed. It is conceivable that these processes are integrated^ in part by control of anion exit from mitochondria to the cytosol. For example, if inhibitor X could be found which blocked
citrate exit but augmented malate exit while fatty acid oxidation was proceeding, then fatty acid biosynthesis would be impaired because of the absence of an acetyl-CoA-generating system in the cytoplasm. The increased rate of fatty acid oxidation, however, would simultaneously generate more mitochondrial acetyl-CoA, leading to enhanced keto-genesis. This would concomitantly create conditions favorable to in
creased rates of carboxylation of pyruvate to oxaloacetate within mito
chondria. The increased amounts of NADH produced would be oxidized to NAD within mitochondria as oxaloacetate was converted to malate prior to the transport of malate to the cytoplasm. The subsequent recon
version of malate to oxaloacetate in the cytoplasmic compartment of liver cells would simultaneously generate NADH, required for the triose phosphate dehydrogenase step, and substrate for the phosphoenolpyruvic carboxykinase reaction, both of which are needed for gluconeogenesis to proceed [for review, see Lardy et al. (88)].
Coordination of other metabolic processes would have to occur to per
mit rapid rates of glucose production, i.e., the blocking of pyruvate kinase activity to avoid operation of a "futile cycle." Knowledge of central control points in these integrative systems is very limited, and we can only guess at present how to evaluate the relative importance of the multiple mechanisms regulating the lipases, the anion transporters, etc., in the control of rates of fatty acid oxidation and gluconeogenesis under different physiological conditions. The availability of inhibitors that would block in vivo either the lipases, the fatty acyl-CoA synthases, or carnitine acyltransferases would go far in defining the importance of each of these separate reactions in the control of fatty acid oxidation.
It would also allow an investigation of which steps of this pathway are required for interdigitation with the gluconeogenesis pathway.
Similarly, the availability of specific in vivo inhibitors of malate or citrate transport across mitochondrial membranes, citrate lyase, acetyl-CoA carboxylase, and fatty acid synthetase would permit an evaluation of the relative regulatory capacity of each reaction in fatty acid bio
synthesis and its relation to the integrative machinery that blocks lipolysis, fatty acid oxidation, and gluconeogenesis when rapid rates of fatty acid synthesis are occurring. The need for the development of specific inhibitors of each of these processes in vivo is obvious. Inhibitors currently available have allowed the elucidation of pathways in isolated systems. Suitable future modification of various classes of these inhibitors will hopefully permit a more careful dissection of the control of discrete steps of fatty acid metabolism in relation to the integration of rates of various metabolic pathways in complex systems.
342 I. B. FRITZ AND M. HALPERIN
ACKNOWLEDGMENTS
Support for work quoted b y I. B . Fritz and M . Halperin was derived from grants from the Medical Research Council of Canada. It is a pleasure to acknowledge the helpful and skillful aid of Mrs. Jackie Campbell in assembling this manuscript.
W e also wish to express our gratitude to Dr. Francis Rolleston for his suggestions for improving the review.
REFERENCES*
1. D . E. Green and D . W . Allman, in "Metabolic Pathways" ( D . M . Greenberg, ed.), 3rd ed., V o l . 2, p. 1. Academic Press, New York, 1968.
2. R. Bressler, in "Lipid Metabolism" (S. J. Wakil, ed.), p. 49. Academic Press, New York, 1970.
3. G. D . Greville and P. K Tubbs, Essays Biochem. 4, 155 (1968).
4. H . R. Mahler and E. H. Cordes, "Biological Chemistry," p. 508. Harper, N e w York, 1966.
5. A. L. Lehninger, "Biochemistry, The Molecular Basis of Cell Structure and Function," p. 417. Worth Publ., New York, 1970.
6. I. B. Fritz, Physiol. Rev. 41, 52 (1961).
7. L. A . Carlson and P. R . Bally, in "Handbook of Physiology" (Amer. Physiol.
S o c , J. Field, ed.), Sect. 5, p. 557. Williams & Wilkins, Baltimore, Maryland, 1965.
8. L. A . Carlson, J. Boberg, and B. Hogstedt, in "Handbook of Physiology"
(Amer. Physiol. S o c , J. Field, ed.), Sect. 5, p. 625. Williams & Wilkins, Baltimore, Maryland, 1965.
9. L. A. Carlson, Atheroscler., Proc. Symp., 2nd, 1969. p. 512 (1970).
10. E. R . Froesch, Diabetologia 3, 475 (1967).
11. A. Hasselblatt, Naunyn-Schmiedebergs Arch. Pharmakol. Exp. Pathol. 262, 152 (1969).
12. L. A. Menahan and O. Wieland, Eur. J. Biochem. 9, 182 (1969).
13. J. Frohlich and O. Wieland, Eur. J. Biochem. 19, 557 (1971).
14. E . W . Sutherland, I. 0 y e , and R . W . Butcher, Recent Progr. Horm. Res.
21, 623 (1965).
15. E . W . Sutherland and G. A. Robison, Diabetes 18, 797 (1969).
16. G. A . Robison, E . W . Sutherland, and E. Butcher, "Cyclic A M P . " Academic Press, New York, 1971.
17. J. K. Huttunen, D . Steinberg, and S. E. Mayer, Biochem. Biophys. Res. Com~
mun. 41, 1350 (1970).
18. J. D . Corbin, E . M . Reimann, D . A. Walsh, and E. G. Krebs, / . Biol. Chem.
245, 4849 (1970).
19. D . A. Walsh, J. P. Perkins, and E. G. Krebs, J. Biol. Chem. 243, 3763 (1968).
20. E. G. Loten and J. G. T . Sneyd, Biochem. J. 120, 187 (1970).
21. G. M . Grodsky, Vitam. Horm. (New York) 28, 37 (1970).
22. L. A. Carlson and L. Oro, Acta Med. Scand. 172, 641 (1962).
23. V . P. Dole, / . Biol. Chem. 236, 3125 (1961).
24. V . P. Dole, J. Biol. Chem. 237, 2758 (1962).
* T o limit the size of the bibliography, it was unfortunately necessary to refer frequently to review articles rather than attempt to cite all original articles.
25. D . Steinberg, M . Vaughan, P. Nestel, and S. Bergstrom, Biochem. Pharmacol.
1 2 , 764 (1963).
26. L. Birnbaumer and M . Rodbell, / . Biol Chem. 2 4 4 , 3477 (1969).
27. W . Schwabe, E. Kerstein, and A. Hasselblatt, N aunyn-Schmiedebergs Arch.
Pharmakol. Exp. Pathol. 2 6 0 , 1 (1968).
28. P. H . Schreibman, D . E. Wilson, and R. A . Arky, Diabetes 2 0 , 146 (1971).
29. L. Galzigna, C. R . Rossi, L. Sartorelli, and D . M . Gibson, J. Biol. Chem.
2 4 2 , 2111 (1967).
30. F. Compagnari and L. T. Webster, / . Biol. Chem. 2 3 8 , 1628 (1963).
31. H . R . Mahler, S. J. Wakil, and R . M . Bock, J. Biol. Chem. 2 0 4 , 453 (1953).
32. J. Bar-Tana, G. Rose, and B. Shapiro, Biochem. J. 1 0 9 , 269 (1968).
33. A . Kornberg and W . E. Pricer, Jr., / . Biol. Chem. 2 0 4 , 329 (1953).
34. M . Barstad, Biochim. Biophys. Acta 1 4 6 , 272 (1967).
35. J. Bar-Tana, G. .Rose, and B. Shapiro, Biochem. J. 1 2 2 , 353 (1971).
36. P. B. Garland, D . W . Yates, and B. A. Haddock, Biochem. J. 1 1 9 , 553 (1970).
37. K . Lippel, J. Robinson, and E . G. Trams, Biochim. Biophys. Acta 2 0 6 , 173 (1970).
38. M . Farstad, J. Bremer, and K . R. Norum, Biochim. Biophys. Acta 1 3 2 , 492 (1967).
39. A. Van T o l and W . C . Hulsmann, Biochim. Biophys. Acta 2 2 3 , 116 (1970).
40. S. V . Pande and J. F. Mead, / . Biol. Chem. 2 4 3 , 352 (1968).
41. M . Aas, Biochim. Biophys. Acta 2 3 1 , 32 (1971).
42. A . Alexandre, C. R . Rossi, L. Sartorelli, and N . Siliprandi, FEBS Lett. 3 , 279 (1969).
43. R . Bressler, C. Corredor, and K. Brendel, Pharmacol. Rev. 2 1 , 105 (1969).
44. C. v o n Holt, M . von Holt, and H . Bohm, Biochim. Biophys. Acta 1 2 5 , 11 (1966).
45. K. Brendel, C. Corredor, and R. Bressler, Biochem. Biophys. Res. Commun.
3 4 , 340 (1969).
46. A . E. Senior, B . Robson, and H . S. A. Sherratt, Biochem. J. 1 1 0 , 511 (1968).
47. J. R . Williamson, S. G. Rostand, and M . J. Peterson, J. Biol. Chem. 2 4 5 , 3242 (1970).
48. N . B. Ruderman, C. J. Toews, C. Lony, I. Vreeland, and E. Shafrir, Amer.
J. Physiol. 2 1 9 , 51 (1970).
49. C. Corredor, K Brendel, and R. Bressler, Proc. Nat. Acad. Sci. U.S. 5 8 , 2299 (1967).
50. A . E . Senior and H . S. A . Sherratt, Biochem. J. 1 1 0 , 499 (1968).
51. P. C. Holland and H. S. A. Sherratt, Biochem. J. 1 1 8 , 4P (1969).
52. H . S. A. Sherratt, Brit. Med. Bull. 2 5 , 250 (1969).
53. K . Tanaka, E. M . Miller, and K . J. Iselbacher, Proc. Nat. Acad. Sci. U.S.
6 8 , 20 (1971).
54. I. B. Fritz, Advan. Lipid Res. 1, 285 (1963).
55. I. B. Fritz, Perspect. Biol. Med. 1 0 , 643 (1967).
56. J. Bremer, in "Cellular Compartmentalisation and Control of Fatty Acid M e tabolism" (F. C. Gran, ed.), p. 65. Academic Press, New York, 1968.
57. I. B. Fritz and K. T. N . Yue, / . Lipid Res. 4 , 279 (1963).
58. B. A. Haddock, D . W . Yates, and P. B. Garland, Biochem. J. 1 1 9 , 565 (1970).
59. D . W . Yates and P. B. Garland, Biochem. J. 1 1 9 , 547 (1970).
60. I. B. Fritz, S. Schultz, and P. Srere, J. Biol. Chem. 2 3 8 , 2509 (1963).
344 I. B. FRITZ AND M. HALPERIN
Fritz, Diabetes 17, 194 (1968).
72. J. F. A. Chase and P. K . Tubbs, Biochem. J. I l l , 225 (1969).
78. I. B. Fritz and E. Kaplan, Protides Biol. Fluids, Proc. Colloq. 7, 252 (1960).
79. P. B . Garland, B. Chance, L. Ernster, C. P. Lee, and D . Wong, Proc. Nat.
85. P. J. Randle, in "Nobel Symposium 13, Pathogenesis of Diabetes Mellitus"
(E, Cerasi and R. Luft, eds.), p. 173. Wiley, New York, 1970. Comp. Physiol. 66, Suppl. 1, 39 (1965).
89. P. Walter, V . Paetkau, and H . A. Lardy, J. Biol. Chem. 241, 2523 (1966).
93. P. A. Mayes, in "Adipose Tissue, Regulation and Metabolic Functions" (B.
Jeanrenaud and D . Hepp, eds.), p. 186. Academic Press, New York, 1970.
94. M . Vaughan, / . Lipid Res. 2, 293 (1961).
95. P. R . Vagelos, A. W . Alberts, and P. W. Majerus, Ann. N.Y. Acad. Sci. 131, 177 (1965).
96. P. J. Randle, R. M . Denton, and M . L. Halperin, in "Physiopathology of Adipose Tissue" (J. Vague, ed.), p. 31. Excerpta Med. Found., Amsterdam, 1969.
97. J. M . Lowenstein, Biochem. Soc. Symp. 27, 61 (1968).
98. E. G. Ball, Advan. Enzyme Regul. 4, 3 (1966).
99. P. Favarger, in "Handbook of Physiology" (Amer. Physiol. S o c , J. Field, ed.), Sect. 5, p. 19. Williams & Wilkins, Baltimore, Maryland, 1965.
100. B. Jeanrenaud and A. E. Renold, in "Physiopathology of Adipose Tissue"
(J. Vague, ed.), p. 20. Excerpta Med. Found., Amsterdam, 1969.
101. A. G. Goodridge and E. G. Ball, Amer. J. Physiol. 211, 803 (1966).
102. G. F. Cahill, Jr., J. Ashmore, A. S. Earle, and S. Zotta, Amer. J. Physiol.
192, 91 (1958).
103. D . G. Walker, Essays Biochem. 2, 33 (1966).
104. E. A. Newsholme and W. Gevers, Vitam. Horm. (New York) 25, 1 (1967).
105. 0 . M . Lowry and J. V. Passonneau, / . Biol. Chem. 241, 2268 (1966).
106. M . E. Krahl, Ann. N.Y. Acad. Sci. 54, 649 (1951).
107. A. I. Winegrad and A. E. Renold, J. Biol. Chem. 233, 267 (1958).
108. B. H. Robinson and M . L. Halperin, Biochem. J. 116, 229 (1970).
109. M . L. Halperin and B. H . Robinson, Biochem. J. 116, 235 (1970).
110. M . L. Halperin, Can. J. Biochem. 48, 1228 (1970).
111. F. J. Ballard and R. W . Hanson, J. Lipid Res. 8, 73 (1967).
112. S. Papa, N . E. Lofrumento, E. Quagliariello, A. J. Meijer, and J. M . Tager, Bioenergetics 1, 287 (1970).
113. B. H. Robinson, G. R . Williams, M . L. Halperin, and C. C. Leznoff, J. Biol.
Chem. 246, 5280 (1971).
114. P. R . Vagelos, Curr. Top. Cell. Regul. 4, 119 (1971).
115. Y . Stein and B. Shapiro, Biochim. Biophys. Acta 24, 197 (1954).
116. J. M . Johnston, Advan. Lipid Res. 1, 105 (1964).
117. J. R . Senior, J. Lipid Res. 5, 495 (1964).
118. J. M . Johnston, in "Handbook of Physiology" (Amer. Physiol. S o c , J. Field, ed.), Sect. 6, V o l . II, p. 1353. Williams & Wilkins, Baltimore, Maryland, 1968.
119. G. Hubscher, D . N . Brindley, M . E. Smith, and B. Sedgwick, Nature (London) 216, 449 (1967).
120. F. M . Schultz and J. M . Johnston,J. Lipid Res. 12, 132 (1971).
121. M . C. Scrutton and M . F. Utter, Annu. Rev. Biochem. 37, 249 (1968).
122. J. V. Passonneau and O. M . Lowry, Biochem. Biophys. Res. Commun. 7, 10 (1962).
123. T . E. Mansour, J. Biol. Chem. 238, 2285 (1963).
124. T. Tanaka, Y . Harano, F. Sue, and H. Morimura, / . Biochem. (Tokyo) 62, 71 (1967).
125. C. I. Pogson, Biochem. J. 110, 67 (1968).
126. E. Bailey, F. Stripe, and C. B. Taylor, Biochem. J. 108, 427 (1968).
127. G. Weber, M . A. Lea, and N . B. Stamm, Life Sci. 6, 2441 (1967).
128. L. J. Reed, Curr. Top. Cell. Regul. 1, 233 (1969).
129. S. Papa, A. Francavilla, G. Paradies, and B. Meduri, FEBS Lett. 12, 285 (1971).
130. T. C. Linn, F. H . Pettit, and L. J. Reed, Proc. Nat. Acad. Sci. U.S. 62, 234 (1969).
346 I. B. FRITZ AND M. HALPERNI 131. T. C. Linn, F. H . Pettit, F. Hucho, and L. J. Reed, Proc. Nat. Acad. Sci.
U.S. 6 4 , 227 (1969).
132. O. Wieland and B. Jagow-Westermann, FEBS Lett. 3 , 271 (1969).
133. R. L. Jungas, Endocrinology 8 6 , 1368 (1970). 13, p. 235. Academic Press, New York, 1969.
140. E. Palacian, G. de Torrontequi, and M . Losada, Biochem. Biophys. Res. Com
mun. 2 4 , 644 (1966).
141. R . E. Eackin, E. E. Snell, and R . J. Williams, / . Biol. Chem. 1 3 6 , 801 (1940).
142. P. A. Srere, in "Methods in Enzymology" (J. M . Lowenstein, ed.), V o l . 13, p. 3. Academic Press, New York, 1969.
143. N . O. Jangaard, J. Unkeless, and D . E. Atkinson, Biochim. Biophys. Acta 1 5 1 , 225 (1968).
144. D . Sheppard and P. B. Garland, Biochem. Biophys. Res. Commun. 2 2 , 89 (1966).
145. G. W. Kosicki and L. P. K . Lee, J. Biol. Chem. 2 4 1 , 3571 (1966).
146. O. Wieland and L. Weiss, Biochem. Biophys. Res. Commun. 1 3 , 26 (1963).
147. P. K. Tubbs, Biochim. Biophys. Acta 7 0 , 608 (1963).
166. C . A. Plate, V. C. Joshi, B . Sedgwick, and S. J. Wakil, J. Biol. Chem. 2 4 3 ,
171. A. F. Rosenthal and S. Ching-Hsien Han, Biochim. Biophys. Acta 1 5 2 , 96 (1968).
176. B. H. Robinson and J. B. Chappell, Biochem. Biophys. Res. Commun. 2 8 , 249 (1967).
181. E. Quagliarello, F. Palmieri, G. Prezioso, and M . Klingenberg, FEBS Lett.
4 , 251 (1969).
182. R. W . Hanson, M . S. Patel, M . Jomain-Baum, and F. J. Ballard, Metab., Clin. Exp. 20, 27 (1971).
183. M . S. Kornacker and E. G. Ball, Proc. Nat. Acad. Sci. U.S. 5 4 , 889 (1965).
184. A. L. Lehninger, / . Biol. Chem. 1 9 0 , 345 (1951).
185. P. Borst, "Funktionelle und morphologische Organization der Zelle," p. 137.
Springer-Verlag, Berlin and New York, 1963.
186. J. D . McGivan, N . M . Bradford, and J. B. Chappell, FEBS Lett. 4 , 247 (1969).
186a. R . Rognstad and J. Katz, Biochem. J. 1 1 6 , 483 (1970).
187. J. D . M c G i v a n and J. B. Chappell, Biochem. J. 1 1 6 , 37P (1970).
188. T . E. Barman, in "Enzyme Handbook," V o l . 1, p. 58 and 355. Springer-Verlag, Berlin and New York, 1969.
189. J. P. Flatt, / . Lipid Res. 1 1 , 131 (1970).
194. J. C. Londesborough and K. Dalziel, in "Pyridine Nucleotide-Dependant Dehy
drogenases" ( H . Sund, ed.), p. 315. Springer-Verlag, Berlin and New York, 1970.
195. T . Dobzhansky, in "Philosophy, Science and M e t h o d : On Cartesian and Dar
winian Aspects of Biology (Essays in Honor of E. N a g e l ) " (S. Morgenbesser, P. Suppes, and M . White, eds.), p. 165. St. Martin's, New York, 1969.