The inhibition of sheep liver arginase by some l-amino-acids

Abstract

1. 1. The effects of eighteen common l-amino-acids on sheep liver arginase were studied. Alanine and histidine showed a slight stimulation of the enzyme activity. Ornithine, lysine, leucine, isoleucine, valine, and proline inhibited the enzyme. Other amino-acids were without any significant effect. 2. 2. Inhibition of the enzyme seems to be a property of the carbon chain length of the amino-acids. Ornithinc, lysine, and proline were competitive inhibitors, while valine showed the mixed type inhibition of the unactivated enzyme. 3. 3. Leucine competitively inhibited the unactivatcd enzyme, but snowed mixed-type inhibition of the enzyme activated by pretreatment with Mn 2+. 4. 4. Isoleucine inhibited the unactivatcd enzyme competitively at high substrate concentrations and non-competitively at low substrate concentrations, suggesting negative heterotropic co-operative effects. 5. 5. These negative co-operative effects of isoleucine, however, were reversed when the enzyme was activated with Mn 2+. 6. 6. These results suggest that activation of arginase with divalent cations alters the kinetics of inhibition of the enzyme by ammo-acids.