Studies on the subsite specificity of the rat brain puromycin-sensitive aminopeptidase

The specificity of the puromycin-sensitive aminopeptidase from rat brain was examined. Using l-alanyl-β-naphthylamide as substrate V_max of the reaction was shown to be pH independent over the range of 5.5-9.0, while K_m exhibited a pK_a of 7.7. This latter value corresponds to the pK_a of the amino group of the substrate. Using X-Ala and X-Leu to examine the specificity of the P1 site it was found that Arg and Lys exhibit the highest affinity, followed by Met, Val, Leu, Trp, and Phe, which bind ∼-5- to 20-fold less well. Although K_m varied more than 20-fold within this series, V_max showed considerably less variation. Significantly weaker binding was observed with a P1 Gly, Ala, Ser, or Pro with no binding detectable with a P1 Glu. The presence of a P′1 Leu compared to P′1 Ala results in an approximate 10-fold decrease in K_m with little change in V_max. The effect of varying P′1 residues was examined with the series Leu-X. In this case basic and hydrophobic amino acids, with the exception of Val, all exhibit nearly the same K_m. The binding of Arg-Arg and Lys-Lys showed the same K_m as obtained for Arg-Leu or Lys-Leu, respectively. When Leu-Ser-Phe was compared to Leu-Ser the P′2 residue led to a 100-fold decrease in K_m and slightly less than a 5-fold increase in V_max. In contrast the addition of a P′2 Met to Leu-Trp results in only a 3-fold decrease in K_m and a 3-fold increase in V_max. The results indicate a preference for a basic or hydrophobic residue in the P1 and P′1 sites and indicate subsite-subsite interactions which primarily affect binding.