Stimulated transport of hypoxanthine in Crithidia luciliae: relationship to purine stress

Abstract

Hypoxanthine transport in the insect trypanosome, Crithidia luciliae, was activated in purine depleted conditions. The existence of 2 hypoxanthine transport mechanisms was established. The first, a non-saturable diffusion system, present in purine replete conditions, exhibited properties that were different from the second transport system which was evident only during purine depleted conditions (purine stress). The rate of transport under purine stress was elevated approximately 8-fold over that in replete conditions. This transporter was saturable with a K_m of 3·9 μM for hypoxanthine. The transporter substrate specificity included other purine bases, e.g. adenine and guanine, and the purine nucleoside, adenosine. These inhibited hypoxanthine transport competitively with K_i values of 2 μM, 3 μM and 42 μM respectively. Coincident with the increase of hypoxanthine transport under purine stress, the transport of adenosine increased 4-fold and the activity of the 3′-nucleotidase ectoenzyme also increased significantly. Under purine stress the concurrent increase of hypoxanthine and adenosine transport and the increase in 3′-nucleotidase activity could be repressed with either the supplementation of excess purines or by cycloheximide. This study of purine salvage mechanisms in Crithidia luciliae illustrates the successful adaptation of the parasite to nutritional insufficiency.