NADP+ reduction by a methanogen using HCOOH or H2 as electron donor

Abstract

The resting cells of methanogen strain HU could be used as biocatalyser for converting exoge nous NADP+ into NADPH, using either formate or hydrogen as electron donor. To enhance the conversion efficiency of NADPH from NADP+, several inhibitors of methylcoenzyme M reductase were used in order to avoid further oxidation of NADPH to CH4. When methyl viologen (7.5 μmol ml-1) was added to the reaction mixture (17 mg of dry cells, 2 mg Triton X-100, 294 μmol of Na-formate and 12 μmol of NADP+ per ml reaction mixture), 9.6 μmol ml-1 NADPH (80% yield) could be produced in a 2-h reaction, compared with 7.2 μmol ml-1 NADPH (60% yield) in a 6-h reaction in the absence of methyl viologen. Molecular hydrogen istead of formate also served as electron donor to convert NADP+ into NADPH. A gas mixture of H2/N2 (75/25) yielded 9.8 μmol ml-1 NADPH (82% yield) in a 3-h reaction in the absence of formate, suggesting that H2 might be a promising, inexpensive electron donor for this reaction system.