This study evaluated the effects of alloxan on the kinetics properties of the delta-aminolevulinate dehydratase (delta-ALA-D) using mouse liver homogenates. delta-ALA-D is an important sulfhydryl enzyme that catalyses the second step in heme biosynthesis and is commonly diminished in experimental and human diabetes. Despite the known effects of alloxan in models of experimental diabetes, there are no data in the literature demonstrating the effects of alloxan on the kinetics properties of the delta-ALA-D. The results showed that alloxan (1.25-20 mu M) caused a concentration-dependent inhibition of hepatic delta-ALA-D activity. The inhibition constant (K (i) ) for alloxan-induced inhibition on delta-ALA-D was 3.64 mu M. The alloxan (5 mu M) caused a decrease in V (max) (65.8%) and in K (m) (53.1%), which is suggestive of an uncompetitive inhibition of enzyme. In addition, dithiothreitol (700 and 1,000 mu M) completely prevented the delta-ALA-D activity inhibition induced by 10 and 20 mu M alloxan. Similar protection was obtained in the presence of 2,000 mu M glutathione. Therefore, this work showed that the inhibition of hepatic delta-ALA-D activity can be obtained in vitro at low micromolar levels of alloxan, and can also be prevented by reducing agents. Moreover, these results may help to understand the abnormalities in heme pathway found in models of experimental diabetes in vivo.