The in vitro degradation of nitroglycerin (NTG) and its dinitrate metabolites in human blood and red blood cells (RBC) has been shown to exhibit apparent first-order kinetics. The decay rates of NTG and its dinitrate metabolites, however, were dependent on the initial concentration. We showed that this unusual kinetic behavior can be described mathematically by models of Michaelis-Menten kinetics combined with either competitive product inhibition or cosubstrate depletion. Experimental studies were conducted to determine the relative contribution of these two mechanisms to the observed kinetics. The effect of added thiols (the likely cosubstrates) on [14C]NTG degradation was studied separately in whole blood, reconstituted RBC, lysed RBC, and plasma. N-Acetylcysteine, L-cysteine, and D-cysteine accelerated NTG degradation in whole blood, while a similar concentration of glutathione had no effect. However, all four thiols exerted no effect on NTG kinetics in reconstituted and lysed RBC. In contrast, these thiols, as well as dithiothreitol, produced a marked increase (3-14 fold) in NTG degradation rate in plasma compared with buffer controls. Since thiol replenishment in reconstituted and lysed RBC did not abolish the concentration dependency, cosubstrate depletion due to thiols appeared unimportant as a contributor to the kinetic phenomenon. In human blood, metabolite inhibition of NTG degradation occurred along with the existence of concentration dependency. Both phenomena, however, were absent when NTG degradation was examined in rat blood. Concentration-dependent degradation in human blood was not observed for glyceryl-1-mononitrate, a compound that does not produce a nitrated metabolite.(ABSTRACT TRUNCATED AT 250 WORDS)