462 2. ANALOGS OF ENZYME REACTION COMPONENTS 



by heparin. It has been known for many years that heparin depresses cell 

 division and it has been said to prevent gelation of the mitotic apparatus. 

 Paff et al. (1952) found that heparin inhibits mitosis in cultured chick heart 

 fibroblasts and after 24 hr there is a marked accumulation of granular ri- 

 bonucleoprotein in the cells. It was postulated that heparin might interfere 

 with the metabolism of nucleoproteins and thereby block mitosis. 



The inhibition of ribonuclease by DNA was first clearly shown by Houck 

 (1957 b) and this could be thought of more reasonably as a true analog 

 inhibition. Likewise, deoxyribonuclease can be inhibited by RNA, the K^ 

 being around 0.00001 vaM for the endonuclease of E. coli (Lehman et al., 

 1962 a, b). Although inhibition occurs with RNA from various sources, the 

 most potent inhibitor is the amino acid acceptor RNA from E. coli, the 

 inhibition being competitive. The other DNA-cleaving enzymes tested are 

 not inhibited. The potency and specificity of this inhibition cannot but 

 stimulate thoughts on the possible regulatory relationships intracellularly. 



Other macroanions have variable effects on ribonuclease. ZoUner and 

 Fellig (1952) reported no inhibition by chondroitin sulfate, hyaluronate, 

 and alginate, but Houck (1957 b) found chondroitin sulfate A and hyalur- 

 onate to inhibit equivalently with heparin. Synthetic polyglucose sulfate is 

 a competitive inhibitor, the effect decreasing with increase in the pH, the 

 net charge on the ribonuclease being reduced (Mora, 1962). Vandendriessche 

 (1956) studied the inhibitions by sulfonated pectin, poly-j9,'/)-dioxydibenzyl 

 phosphate, and poly-L-aspartate, and found them to be much weaker than 

 heparin, while Fellig and Wiley (1959) claimed that the sulfation of a va- 

 riety of polysaccharides (e.g., cellulose, amylose, amylopectin, dextran, pec- 

 tate, and nitrochitin) produces inhibitors often more potent than heparin 

 (although in this work the inhibition by heparin was unaccountably weak), 

 Ribonuclease is also inhibited by copolymers of glutamate and tyrosine (or 

 phenylalanine), which are more affective than poly aspartate or poly gluta- 

 mate (Sela, 1962). Interactions between the benzene rings of the aromatic 

 amino acids and certain groups on the enzyme were believed to occur in 

 addition to the electrostatic forces. Possibly the most potent inhibitors 

 were discovered by Heymann et al. (1958) in a survey of 66 macroanions 

 of synthetic origin, some inhibiting around 50% at 0.001 mg/ml. It was 

 noted that the inhibitory activity is markedly reduced in the presence of 

 proteins, a point worth considering in the use of such substances in cellular 

 preparations. A number of these polymers exhibit antiviral activity against 

 influenzal and vaccinial infections in eggs. Sulfate groups seem to be par- 

 ticularly able to confer inhibitory activity on polymers and in the sulfated 

 polysaccharides the carboxylate groups may be relatively unimportant, 

 since Dickman (1958) showed that sulfation of pectate, pectate methyl 

 ester, and pectic amide gives inhibitors roughly equiactive. It should finally 

 be noted that ribonucleases of different origins may not be equally suscep- 



