Guinea-pig liver morphine 6-dehydrogenase as a naloxone reductase

Elution profiles of guinea-pig liver naloxone reductase and morphine 6-dehydrogenase on Matrex green A, Sephadex G-100 and DEAE-cellulose (DE32) column chromatography used sequentially in the purification procedure were identical. The ratios of the two enzyme activities were almost constant throughout all the purification steps. The two enzymes were similarly more stable at pH 6.0 than at pH 8.0 on storage at 4 degrees. The reversible inactivation of the two enzymes by the removal of 2-mercaptoethanol from the enzyme solution was the same. Inhibitory effects of lithocholic acid, CuSO4, quercitrin, phenylarsine oxide, and prostaglandin E1 on the two enzymes were almost the same. These results indicated that naloxone reductase is identical to morphine 6-dehydrogenase in the guinea-pig liver. For the reduction of naloxone, the enzyme utilized either NADPH or NADH as cofactor, and pH optima were 6.8 with NADPH and 6.2 with NADH. The Km values for NADPH and NADH were 6.5 and 2.2 microM respectively. The Vmax values for naloxone were 1.2 units/mg protein with NADPH and 0.5 unit/mg protein with NADH. The Km values for naloxone were 0.27 mM with NADPH and 0.44 mM with NADH. The reaction product formed by the enzyme was identified as 6 alpha-naloxol by thin-layer and gas-liquid chromatographic analyses. Accordingly, it is clear that the enzyme catalyzes the stereospecific reduction of naloxone to form the 6 alpha-hydroxyl congener.