Aspartate transcarbamylase from Leishmania donovani. A discrete, nonregulatory enzyme as a potential chemotherapeutic site

Abstract

Leishmania donovani is a protozoal pathogen that belongs to the kinetoplastida order. Unlike in other eucaryotic systems, the first three enzymes of the de novo pyrimidine biosynthetic pathway are not components of a multifunctional protein system. The three enzyme activities in the crude extract were separated on a Sephacryl S-200 column. Aspartate carbamoyltransferase (EC 2.1.3.2) has been purified to apparent homogeneity. The enzyme has an approximate molecular weight of 135,000 and seems to be a tetramer of equivalent subunits of molecular weight 35,000. The enzyme shows strictly hyperbolic kinetics with both the substrates under a variety of conditions and is not inhibited by nucleotide phosphates. K_m for carbamyl phosphate is 3.1 × 10^-4 M and for aspartate is 7.6 × 10^-3 M. Apparently, the enzyme has no regulatory role in pyrimidine biosynthesis. N-(Phosphonoacetyl)-L-aspartic acid is a powerful competitive inhibitor (K_i = 5 × 10^-7 M) for this enzyme with carbamyl phosphate as substrate. This inhibitor completely inhibits the growth of the vector form of organism at 60 μM and significantly affects the growth of the pathogenic form in a macrophage assay system. The potency of the inhibitor is comparable with allopurinol which is undergoing human clinical trial as an antileishmanial drug.