Aspirin survival in human blood modulated by the concentration of erythrocytes.

In vitro aspirin hydrolysis rates were measured in fresh human whole blood and in its separate components. The half-life of aspirin in whole blood was relatively rapid (mean 22.2 +/- 3.9 minutes) and exhibited a significant negative correlation with hematocrit (r = -0.96). Hydrolysis of aspirin in buffer that contained only washed red blood cells (40%) was significantly more rapid (mean half-life 17.5 +/- 2.0 minutes) than that in whole blood. When aspirin was incubated in solutions of washed red blood cells that contained human serum albumin in various concentrations, the aspirin half-life was found to vary directly with the concentration of albumin used; at normal levels of albumin, the hydrolysis rate of aspirin approximated that measured in whole blood. The presence of diisopropyl fluorophosphate in low concentrations (0.02-0.05 mM) markedly inhibited the rate of aspirin hydrolysis in washed red blood cells and whole blood. We conclude that enzymes(s) linked to the erythrocyte, probably membrane-bound acetylcholinesterase, control the survival of aspirin in blood.