Allopurinol transport in human erythrocytes

The mechanism of allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidine] transport into human erythrocytes was investigated with an inhibitor stop assay. Allopurinol transport could be resolved into two components: (1) a saturable system and (2) a non-saturable process, which most likely represents non-facilitated diffusion. Allopurinol transport had a Km of 268 mumol/L and a Vmax of 28 pmol/microL intracellular volume/sec; the non-saturable component was 0.0195/sec. Mutual inhibition studies showed that the competitive Ki values of hypoxanthine and adenine on allopurinol transport were 120 and 3 mumol/L, respectively. These Ki values as well as the IC50 values of 100-150 mumol/L for hypoxanthine and 3-10 mumol/L for adenine were similar to the corresponding transport Km values of these bases, which are 128 and 8 mumol/L, respectively. The Ki of allopurinol on hypoxanthine transport was 274 mumol/L and thus nearly identical to its Km. Thus in erythrocytes the uricostatic agent allopurinol is an alternative substrate for the purine transport system, but lacks the exceptional high affinity it has for xanthine oxidase. This could explain the paradoxical clinical side effect of allopurinol, namely that it can provoke an attack of gout. Theophylline, a methylated purine, inhibited allopurinol transport with an IC50 of 200-400 mumol/L. Oxypurinol [4,6-dihydroxypyrazolo(3,4-d)pyrimidine], the main metabolite of allopurinol, also inhibited allopurinol transport with an IC50 of 20-40 mumol/L. This is noteworthy, since allopurinol and oxypurinol do not share the same transport system in the kidney.