Synthetic techniques now available have provided homopolymers and copolymers of many of the natural and unnatural α-amino acids, with molecular weights varying from several hundred to several million (274~
277). In 1959 Sela and Katchalski (286) wrote a thorough review of the biological properties of these synthetic polypeptides, including their in
hibitory properties. Several of these polypeptides inhibit certain enzymes, blood clotting processes, viruses, and bacteria, primarily in vitro. Although these various biological antagonisms derive from the polyelectrolytic character of basic and acidic synthetic polypeptides and cannot be classi
fied at present as antimetabolite effects, a brief discussion is included here.
The reader is referred to the excellent review by Sela and Katchalski (286) for further details.
A. Enzyme Inhibition
The attraction between highly charged polypeptides and enzymes is due, in most instances, to nonspecific electrostatic forces between the inhibitor and the enzyme.
The basic polypeptides, polylysine, polyornithine, and poly-p-amino-phenylalanine, inhibit the proteolytic activity of pepsin. The neutral polyalanine and the acidic polyaspartic and polyglutamic acids are not inhibitory. The inhibition of pepsin by polylysine is reversed by the strongly acidic polysaccharide, heparin, possibly as follows:
Pepsin · polylysine+hepar in—*pepsin+polylysine · heparin
Trypsin and ribonuclease are inhibited by acidic polypeptides. Poly
ornithine reverses the inhibition of ribonuclease by polyaspartic acid.
Poly-D-lysine inhibits trypsin when poly-L-lysine is the substrate for the enzyme.
B. Inhibition of Blood Coagulation
Basic polypeptides, such as poly-L-lysine, lysine, and poly-DL-ornithine, but not neutral or acidic polypeptides, retard the clotting of human blood in vitro by inhibiting thrombin formation in blood and plasma. This antithromboplastic or anticoagulant effect of polylysine could be neutralized by heparin or acidic polyamino acids. Conversely, the anticoagulant activity of heparin could be antagonized by polylysine and other basic polypeptides, but not by neutral or acidic polyamino acids.
The acidic polymers, polyaspartic, polyglutamic, and polycysteic acids, retard the clotting of fibrinogen by thrombin, but the basic polypeptides, polylysine, polyarginine, and polyornithine, accelerate the process.
Inhibition of fibrinolysis by polylysine is antagonized by heparin or polyaspartic acid.
C. Antiviral Properties
The infectivity of tobacco mosaic virus is inhibited by polylysine, ap
parently through the formation of ionic bonds in a reversible manner, and also by polyglutamic acid and other acidic polyelectrolytes. In the latter instance, reduction of virus infectivity is thought to be due to an effect upon the host, wherein basic virus receptor sites on the leaf cells are in
activated.
Polylysine has been reported to inhibit influenza, infectious bronchitis, Newcastle, and mumps viruses in the chick embryo.
The inactivation of the bacteriophage of Clostridium madisonii by poly
lysine has been reported. The inactivation of E. coli phage T^hr +
by poly
lysine seems to occur in two steps, a reversible stage, due to electrostatic interaction between the negatively charged phage and the positively charged polyelectrolyte, and an irreversible stage, representing damage to the D N A of the phage.
D. Antibacterial Properties
The antibacterial properties of the basic synthetic polyamino acids, such as polylysine, polyornithine, and polyarginine, resemble those of the naturally occurring basic polypeptides and proteins (cf. Section I I I , E, 2) in inhibiting growth, reducing oxygen uptake, agglutinating, and reducing the electrophoretic mobility of various microorganisms.
Polylysine, but not the neutral or acidic polypeptides, inhibits the growth of certain bacteria in vitro, including E. coli and M. aureus. In
hibitory activity increases with increasing chain length, and is reversed by acidic polyelectrolytes, including polyaspartic and polyglutamic acids.
Certain basic polyamino acid copolymers related to the cyclic poly
peptide antibiotics (cf. Section I I , B) exhibit antibacterial properties. In a structure-antibacterial study of linear peptide analogues of gramicidin S, it was found that the linear decapeptide corresponding to gramicidin S is inhibitory and that antibacterial activity is dependent upon the ornithine or the ornithine-leucine content of these peptides (cf. Section I I , B, 7).
The antimicrobial properties of polylysine have been the subject of other reports (278-281). Inhibitory activity against certain fungi patho
genic for plants and certain fungi pathogenic for man has been described (279). The same authors (279), in studies concerned with tomato wilt, observed that polylysine was toxic for both the host plant and the etio
logical agent of the disease.
Buchanan-Davidson, Seastone, and Stahmann (280, 281) have de
scribed the effects of polylysine on a number of species of gram-positive bacteria. The agglutination of staphylococci, streptococci, and pneumo-cocci also was observed, as was the precipitation of the capsular poly
saccharide of pneumococci and umbilical cord hyaluronic acid. This polypeptide varied in its effect on different species of bacteria, viz., in
creasing the in vitro phagocytosis of pneumococci, reducing the phago
cytosis of staphylococci, and having little effect upon the phagocytosis of streptococci by human leucocytes. The variability of effects suggests that it is unlikely that polylysine exhibits a single mechanism of action; such biological activities probably are the results of a combination of several different mechanisms which are related to molecular size and configura
tion, electrostatic potential, the acidic or basic nature of the (bacterial) cell surface, etc.
In a second paper (281), Buchanan-Davidson et al. report the protective effect of polylysine against Diplococcus pneumoniae infection and against the spores of B. anthracis in mice treated intraperitoneally simultaneously with intraperitoneal infection. In similar experiments, polylysine had no such protective effect against streptococcus and staphylococcus infection, and enhanced Salmonella typhimurium infection in mice. These in vivo data, with the exception of the enhancing effect on S. typhimurium, may be correlated roughly with the effects of polylysine on the phagocytosis of these bacterial species in vitro (280). In the case of B. anthracis infection, the effect of polylysine may be the neutralization of the "aggressin"
activity of natural poly-d-glutamic acid (282), resulting in enhanced phagocytosis of the spores. Another natural polypeptide, calf thymus
histone, exhibits a similar protective effect against infection with B. anthra
cis spores in mice (255) (cf. Section I I I , E, 2).
The synthetic polypeptides exhibit other biological properties, such as interaction with protein, nucleic acids, peripheral blood cells, etc., (236).
A number of these compounds have been studied immunologically (283, 284), and there is one brief report describing the antitumor activity of polylysine against certain mouse ascites tumors (285).
In 1962, subsequent to the review by Sela and Katchalski (236), the proceedings of an international symposium on polyamino acids, poly
peptides, and proteins were published (277). Of particular pertinence to this chapter is the section concerned with biological inhibition by syn
thetic poly-a-amino acids. In this volume (277) Stahmann reviewed chemotherapeutic applications of synthetic polyamino acids against viruses, bacteria, fungi, and experimental tumors, as well as the antigenic, hematological, and hormone-binding properties of these polypeptides;
Cochran and Stahmann reported the partial protection by polylysine of mice infected with poliomyelitis; Sela, Maurer, and Gill and Doty dis
cussed antigenic and immunological properties of polyamino acids; and Blout, Farber, Fasman, Klein, and Narrod reported recent hematological and plasma-expander studies on copolypeptides of glutamic acid and L-lysine.
V. ANALOGUES OF THE POLYPEPTIDE AND PROTEIN HORMONES