Biochemical Characterization of Molybdenum-Cofactor in a Cyanobacterium, Nostoc muscorum

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Cell-free extracts of nitrate-grown Nostoc muscorum contain nitrate reductase and molybdenum-cofactor activities. While nitrate reductase activity is associated with the particulate fraction, cofactor activity is found predominantly in the soluble fraction. This activity was distributed between two pools. In one pool, the molybdenum-cofactor is associated with a carrier (protein) of approximately 30,000 Da with an S_<20,w> between 2.3 and 2.5. The carrier-bound cofactor is non-dialyzable and is found along with the major proteins during filtration in Sephadex G-25 and G-100. The second pool contains free or unbound cofactor. It is separated from soluble proteins by dialysis with a membrane with a pore-size of 10 to 15 kDa. However, it is retained with a membrane with a pore size of 1 kDa. It is in the included volume during chromatography through Sephadex G-25. Its molecular mass is estimated to be between 1,000 and 5,000 Da. The molybdenum content was proportional to cofactor activity in both pools. Reducing agents increased cofactor activity. However, activity in both pools was sensitive to heat, acid, and oxidative treatments. The carrier protein appears to give some protection.
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