Two types of pyridine nucleotide transhydrogenases have been reported in living organisms. The energylinked transhydrogenase is found in mitochondria and in certain heterotrophic and photosynthesizing bacteria, while the non-energy-linkcd transhydrogenase is found in certain heterotrophic bacteria. The presence of a structurally similar non-energy-linked transhydrogenase in A-otohacter vinelandii, Pseudomonas aeruginosa and Pseudomoizas fluorescen.r is readily shown in extrach Tram these bacteria with Western (protein) blotting. This non-energy-linked enzyme is lacking in E.scherichia coli, while the presence of a structurally similar energy-linked enzyme in E. coli and in beef heart mitochondria is indicated with the Western blotting technique. Spinach (Spinacia oleracra) lacks the non-energy-linked transhydrogenase occurring in bacteria. The chloroplast enzyme ferredoxin: NADP+ oxidoreductase, which exhibits non-energy-linked transhydrogenase activity, is immunologically distinct from the bacterial transhydrogenases.In order to provide a rationale for the distribution of the two types of pyridine nucleotide transhydrogenases, the steady-state degrees of reduction of the NADP(H) and NAD(H) pools in A . vinelandii ( R~A D~( H ) and R h A D ( H ) ) have been measured for cells metabolizing sucrose at a variable oxygen flux (&I2). under steadystate conditions explains the energy requirement in the latter reaction. It is found that the degree of reduction of the NADP(H) pool is always higher than that of the NAD(H) poo! ( R~A D~( H ) > RhaucH,) except when 402 goes to zero (R;VADI'(H) z R~A D ( " ) ) . Comparison of these results with literature values indicates that the inequality R h A D P ( H )>The dependence of the non-energy-linked transhydrogenase activity of ferredoxin : NADP' oxidoreductase on RhADp(,{) is compared with that of A . vinelandiii transhydrogenase. The results indicate that this activity is unlikely to be of physiological importance in plant chloroplasts.

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