Leucine aminopeptidase (bovine lens). The relative binding of cobalt and zinc to leucine aminopeptidase and the effect of cobalt substitution on specific activity.

Prolonged incubation of zinc-zinc leucine aminopeptidase (bovine lens) (EC 3.4.1.1) with 0.05 M CoCl2 and M KCl in 0.2 M N-ethylmorpholine-HCl at pH 7.5 and 37 degrees yields an active enzyme in which 2 g atoms of Co2+ per 54,000 dalton subunit have replaced the Zn2+. Incubation of cobalt-cobalt leucine aminopeptidase with various AnCl2 concentrations or zinc-zinc leucine aminopeptidase with various CoCl2 concentrations in M KCl and 0.2 M N-ethylmorpholine-HCl at pH 7.5 and 37 degrees demonstrates that Co2+ and Zn2+ compete reversibly for two independent binding sites per subunit for which the ratio of the association constants for Zn2+ and Co2+ (1KZn:1KCo = 1KZn/Co; 2KZn:2KCo = 2KZn/Co) are 115 and 15.9 for sites 1 and 2, respectively. The specific activities of the various species of enzyme with 2 mM L-leucine p-nitroanilide as substrate in 0.2 M N-ethylmorpholine-HCl and 0.01 M NaHCO3 at pH 7.5 are estimated to be (in micromoles per min per mg) 0.043 for the zinc-zinc. 0.039 for the zinc-cobalt, 0.541 for the cobalt-zinc, and 0.536 for the cobalt-cobalt forms, which implies that activity is affected only when cobalt is substituted at site 1, the "activation site." The site, at which cobalt substitution has no effect on activity, is designated the "structural site." The value of Km for cobalt-cobalt leucine aminopeptidase with L-leucine p-nitroanilide as substrate in 0.2 M N-ethylmorpholine-HCl at pH 7.5 containing 0.01 M NaHCO3 at 30 degrees is 0.52 mM while Vmax is 0.90 mumol per min per mg. In the additional presence of 1 M KCl, Km is 0.19 mM while Vmax is 0.68 mumol per min per mg.