Aerobic purification of N5,N10-methylenetetrahydromethanopterin dehydrogenase, separated from N5,N10-methenyltetrahydromethanopterin cyclohydrolase, from Methanobacterium thermoautotrophicum strain Marburg

Abstract

The N5,N10-methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum strain Marburg has been purified with reasonable yield and much higher specific activity than previously reported. For the first time it has been shown that both N5,N10-methylenetetrahydromethanopterin dehydrogenase and N5,N10-methenyltetrahydromethanopterin cyclohydrolase activities were stable under air and could be purified using aerobic operations. The dehydrogenase activity from Methanobacterium thermoautotrophicum Marburg was stable in phosphate buffer with or without glycerol or ammonium sulfate under both aerobic and anaerobic conditions. However, the presence of either 2-mercaptoethanol or dithiothreitol in the enzyme solution destroyed the enzyme activity during both aerobic and anaerobic incubations. Dehydrogenase was purified 62-fold using Phenyl-Sepharose and DEAE-Sephadex chromatography in succession under air. Both of these chromatographic methods separated dehydrogenase activity from N5,N10-methenyltetrahydromethanopterin cyclohydrolase; DEAE-Sephadex provided the best separation. Phenyl-Sepharose chromatography of the supernatant of cell extracts containing ammonium sulfate at 60% of saturation provided a 4.7-fold purification and 98% recovery of cyclohydrolase; this result established the air stability of N5,N10-methenyltetrahydromethanopterin cyclohydrolase from Methanobacterium thermoautotrophicum Marburg.Key words: methylenetetrahydromethanopterin dehydrogenase, methenyltetrahydromethanopterin cyclohydrolase, Methanobacterium, aerobic purification, oxygen stability.