Evidence for the presence of two pyridine nucleotide-binding sites on the beta subunit of the Escherichia coli pyridine nucleotide transhydrogenase.

The pyridine nucleotide transhydrogenase of Escherichia coli is composed of two types of subunits, alpha and beta. Trypsin digestion of the purified enzyme generates fragments of the alpha subunit. The beta subunit is uncleaved unless NADP(H) is present (Tong, R.C.W., Glavas, N.A. and Bragg. P.D. (1991) Biochim. Biophys. Acta 1080, 19-28). Purified transhydrogenase bound to either NAD- or NADP-agarose was treated with trypsin. The alpha subunit was cleaved to 16, 29 and 43 kDa fragments in both cases. The beta subunit remained bound to NAD-agarose but was released as two cleavage fragments (25 and 30 kDa) from NADP-agarose. The beta subunit of the transhydrogenase bound to NAD-agarose was cleaved by trypsin in the presence of NADP(H) to yield 25 and 30 kDa fragments of the beta subunit. These results suggest that the beta subunit contains two pyridine nucleotide-binding sites.