Occurrence and some properties of a protein-like inhibitor of dopamine beta-hydroxylase in rat liver.

Evidence of endogenous inhibitors of dopamine β-hydroxylase [3, 4-dihydroxyphenylethylamine, ascorbate:O₂ oxidoreductase (hydroxylating), EC 1.14.2.1] (DBH) in rat liver was obtained. At least two kinds of endogenous inhibitors were present in the 105,000 g supernatant of rat liver homogenate and their molecular weights were shown to be about 40,000 and 1200 by gel filtration.The higher molecular weight inhibitor was partially purified by ammonium sulfate fractionation, column chromatography of Sephadex G-100 or Sephadex G-200, and DEAE-Sephadex(A-25) treatment. The activity of this inhibitor was not diminished by boiling for 5 min, but was lost completely when the inhibitor was treated with trypsin, suggesting that it may be a protein. The inhibition of DBH by this protein-like inhibitor in the liver was completely protected by the addition of N-ethylmaleimide, indicating that it may contain functional sulfhydryl groups. The inhibition by this protein-like inhibitor was of the noncompetitive type with both the substrate and with ascorbic acid, one of the cofactors in this reaction.