Purification and Properties of Biliverdin Reductases from Pig Spleen and Rat Liver1

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Abstract
Biliverdin reductase was purified from pig spleen soluble fraction to a purity of more than 90% as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme was a monomer protein with a molecular weight of about 34, 000. Its isoelectric point was at 6.1–6.2. The enzyme was strictly specific to biliverdin and no other oxidoreductase activities could be detected in the purified enzyme preparation. The purified enzyme could utilize both NADPH and NADH as electron donors for the reduction of biliverdin. However, there were considerable differences in the kinetic properties of the NADPH-dependent and the NADH-dependent biliverdin reductase activities: Km for NADPH was below 5 µm while that for NADH was 1.5–2 mm; the pH optimum of the reaction with NADPH was 8.5 whereas that of the reaction with NADH was 6.9; Km for biliverdin in the NADPH system was 0.3 µM whereas that in the NADH system was 1–2 µm. In addition, both the NADPH-dependent and NADH-dependent activities were inhibited by excess biliverdin, but this inhibition was far more pronounced in the NADPH system than in the NADH system. IXbα-biliverdin was the most effective substrate among the four biliverdin isomers, and the di-methylester of IXalpha;-biliverdin could not serve as a substrate. Biliverdin reductase was also purified about 300-fold from rat liver soluble fraction. The hepatic enzyme was also a monomer protein with a molecular weight of 34, 000 and showed properties quite similar to those of the splenic enzyme as regards the biliverdin reductase reaction. The isoelectric point of the hepatic enzyme, however, was about 5.4. It was assumed that NADPH rather than NADH is the physiological electron donor in the intracellular reduction of IXα-biliverdin. The stimulatory effects of bovine and human serum albumins on the biliverdin reductase reactions were also examined.