In vitro binding and degradation of avian pancreatic polypeptide by chicken and rat tissues

The interaction of pancreatic polypeptide (PP) with possible chicken and rat target tissues was investigated by characterizing the binding and degradation of [¹²⁵I]iodo-PP by plasma membrane preparations in vitro. Membranes from chick brain and liver possessed highly specific avian PP (APP)-binding sites, while those from chick whole pancreas and proventricular and duodenal mucosa exhibited little or no specific [¹²⁵I]iodo-APP binding. The affinity of the specific chick liver binding sites for APP was low; 500 ng unlabeled APP/ml (1.2 × ⁻⁷ M) were required for half-maximal displacement of[¹²⁵I]iodo-APP. Chick brain membranes, on the other hand, possessed two orders of APP binding sites, a high affinity site(K_d = 3.3 × 10⁻¹⁰ M) and a low affinity site (K_d = 1.8 × 10⁻⁷ M). The binding process to chick brain membranes retained specificity for intactAPP_1-36, as unlabeled bovine PP_1-36 (BPP_1-36) inhibited specific binding of [¹²⁵]iodo-APP by 50% at a concentration of 7 × 10⁻⁹M (10 times the IC₅₀ level of unlabeled APP). Carboxy-terminalpentapeptides of APP and BPP (APP_32-36 and BPP_32-36) interacted with the chick brain membrane APP-binding sites, but did not possess the full binding activity of the intact molecule. Membranes from rat brain exhibited little APP-specific binding and no BPP-specific binding. Chick kidney membranes degraded more [¹²⁵]iodo-APP than any other chicken tissue. The degradation process was specifically inhibited by unlabeled APP and yielded reaction products of lower molecular weight than intact APP. The antiprotease bacitracin was capable of virtually complete degradation inhibition, but its presence failed to increase APP binding by kidney membranes. It is concluded that chick brain possesses high affinity APP-binding sites, potentially functional at physiological concentrations of the polypeptide. APP-binding sites on liver membranes are probably physiologically nonfunctional, while the kidney is most active relative to other tissues in the degradation and, probably, clearance of APP.