Tryptophan hydroxylase and aromatic l-amino acid decarboxylase activities in the tissues of adult Ascaris suum

Abstract

Serotonin (5-hydroxytryptamine, 5-HT) has been shown to modulate carbohydrate metabolism in adult female Ascaris suum. Two enzymes, l-tryptophan hydroxylase (TOH) and aromatic l-amino acid decarboxylase (AADC), which are responsible for the biosynthesis of 5-HT in mammalian systems, were demonstrated in isolated muscle and intestinal tissue of adult female A. suum. The specific activities of TOH in muscle and intestine were 0.22 nmol min −1 g −1 and 1.48 nmol min −1 g −1, respectively. The apparent Michaelis-Menten constant ( Km) and maximum initial velocity ( V max) of TOH for l-tryptophan (TRP) in the intestine were found to be 21.50 μm and 2.30 nmol min −1 g −1, respectively. The specific activities of AADC in the isolated muscle and intestinal tissues were 1.64 nmol mm −1 g −1 and 5.72 nmol min −1 g −1, respectively. The apparent Km and V max of the muscle AADC for l-5-hydroxy-tryptophan (5-HTP) were 10 μM and 3.40 nmol min −1 g −1. The apparent Km and V max of the intestinal AADC for 5-HTP were 1.08 μ M and 14.50 nmol min −1 g −1, respectively. The apparent inhibitor constant ( Ki) of TOH for p-chlorophenylalanine, an inhibitor of mammalian TOH activity, in isolated intestine of A. suum was 1.72 mM. The TOH activity from A. suum intestine was inactivated upon exposure to oxygen and this inactivation could be partially reversed by anaerobic incubation in the presence of dithiothreitol and ferrous sulfide at 25 ° C. Collectively, the data demonstrated the presence of TOH and AADC activities in adult A. suum isolated muscle and intestinal tissues.