A patient in Kurobe, Japan, was previously reported to have a new class of hereditary methemoglobinemia, type III. In this patient, NADH cytochrome b5 reductase deficiency was observed in lymphocytes and platelets as well as in erythrocytes, but this was not associated with mental retardation. A base change was identified in the gene for NADH cytochrome b5 reductase, causing an amino acid substitution from Leu-148 to Pro. In the present study, the mutant enzyme (Leu-148-->Pro) was expressed in Escherichia coli, purified, and characterized. The mutant enzyme retained about 60% of the catalytic activity of the wild type, but was remarkably heat unstable. By incubating the mutant enzyme at 42 degrees C for 10 minutes, 80% of the enzyme activity was lost, whereas the wild-type enzyme lost < 20% activity after incubation at 50 degrees C for 30 minutes. Another mutant in which Leu-148 was replaced by Ala was prepared to establish the role of the residue. This mutant was apparently less heat stable than the wild type, implying a structural role for Leu-148. Reinvestigation of the enzyme activity in the blood cells and fibroblasts of the type III Kurobe patient, revealed that about 40% of the normal activity was detected in these cells, in contrast to the previous report. Thus, this patient reported previously as having hereditary meth-hemoglobinemia type III was shown to have type I.