The occurence of a nitrate reductase inactivating factor in extracts of Spirodela polyrrhiza

Abstract

NADH-nitrate reductase (NR) had a low activity immediately after extraction from 14 day-old cultures of Spirodela polyrrhiza with the basic extraction solution (50 mM K-phosphate buffer, pH 7.5, 5 mM cysteine and 3 mM EDTA) whereas nitrite reductase (NiR) activity in the same extract was very high. Increasing the EDTA or cysteine concentration or replacing cysteine by dithiothreitol had no effect on the level of NR activity. The addition of 3% bovine serum albumine (BSA) and 0.4 mM phenylmethyl sulfonyfluoride (PMSF) or 3% BSA and 1.5% polyvinylpolypyrrolidone (PVPP) to the extraction solution greatly increased the NR activity. The enzyme activity increased 40 times in the presence of 1.5% PVPP with 3% BSA and 0.4 mM PMSF. The highest NR activity (over 100-fold) was found in the presence of 3% caseine. The probable nature of the factor inactivating nitrate reductase in S. polyrrhiza extracts is discussed.