Surface aminopeptidase in Moniliformis dubius and its relation to amino acid uptake

Abstract

Surface aminopeptidase (APase) in the acanthocephalan, Moniliformis dubius, was studied using leucylleucine, leucylglycylglycine, glycylglycine, alanylalanine, tri-alanine and tetra-alanine as substrates. In the presence of intact worms all peptides, except glycylglycine, were hydrolysed liberating amino acids and/or peptide subunits into the incubation medium. In 5 min incubations with 5mM leucylleucine, 93% of the liberated leucine was absorbed by the worms indicating a kinetic advantage for absorption of the leucine. In media with leucylleucine plus 10 mM methionine as an inhibitor of leucine uptake, the liberated leucine was not absorbed by the worms, but accumulated in the incubation medium. The inhibitory effects of leucylleucine and alanylalanine (or products of hydrolysis) on the uptake of 0·1 mM [14C]leucine in 2 min incubations were examined. Inhibition of [14C]leucine uptake by these peptides was non-linear up to 5 mM inhibitor concentration. Concentrations of either peptide greater than 5 mM did not produce further inhibition. A significant portion of [14C]leucine transport was not inhibited by these peptides. Inclusion of 0·5 mM Pb2+, an APase inhibitor, blocked the inhibition of [14C]leucine uptake by leucylleucine indicating that the inhibition is caused by the liberated leucine alone, and that interaction of the intact dipeptide and the leucine transport system is negligible. It is concluded that the surface membrane of adult worms has APase which can hydrolyse peptides in the ambient medium. The spatial arrangement of the APase and the leucine transport system is such that the hydrolysis of a peptide confers a kinetic advantage for absorption of the liberated amino acids. Encysted or mechanically excysted cystacanth larvae from the haemocoele of the cockroach showed no APase activity. Activation of the APase required a 30 min treatment in lipase, [Na+]taurocholate or other surface active agents. Since the APase in adult worms was unaffected by washing, the data strongly suggest that this enzyme is of parasitic origin.