β-2-THIENYL-DL-ALANINE (Thi) AS AN INHIBITOR OF PHENYLALANINE HYDROHYLASE (PheH) AND PHENYLALANINE (Phe) INTESTINAL TRANSPORT, IN VITRO AND IN VIVO

Abstract

Thi inhibits phe tubular resorption and alao causes malabaorption of Phe in animals. Structural similarities between Phe and Thi suggested Thi may be a good inhibitor of PheH and hence reproduce the biochemical lesion in PKU. The effects of Thi on rat liver (Lv) and kidney (Kd) PheH were assessed in vitro and in vivo, and on intestinal transport of Phe. The Km for Lv PheH changed from 0.54 mM in the absence of Thi, to 6.6 mM in the presence of 24 mM Thi with no significant change in Vm. For Kd the respective Km were 0.60 and 3.0 mM, both cases inSicating competitive inhibition. Ki were similar in both tissues: 3.2 and 3.4 mM. Hill coefficients close to 1 showed PheH in Lv and Kd was not an allosteric enzyme. No inhibition of PheH in vivo was observed in Lv and Kd, 1 or 24 hr after an i.p. dose of Thi (2 mmolea/kg). When injections were repeated daily for 4 days, only a marginal inhibition of PheH (14% in Lv and 23% in Kd) was observed. In other experiments, when 24 mM Thi was perfused in vivo together with Phe through a 20 cm jejunal segment, it produced a moderate inhibition of Phe intestinal transport. Kinetic studies also indicated competitive inhibition. The elevated Ki for Thi (80 mM)makes it unlikely that oral Thi could effectively decrease Phe absorption and hence provide an alternative to low Phe diets in the management of PKU.