Why are two different types of pyridine nucleotide transhydrogenase found in living organisms?

Two types of pyridine nucleotide transhydrogenases have been reported in living organisms. The energy-linked transhydrogenase is found in mitochondria and in certain heterotrophic and photosynthesizing bacteria, while the non-energy-linked transhydrogenase is found in certain heterotrophic bacteria. The presence of a structurally similar non-energy-linked transhydrogenase in Azotobacter vinelandii, Pseudomonas aeruginosa and Pseudomonas fluorescens is readily shown in extracts from these bacteria with Western (protein) blotting. This non-energy-linked enzyme is lacking in Escherichia 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 oleracea) 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'NADP(H) and R'NAD(H)) have been measured for cells metabolizing sucrose at a variable oxygen flux (phi O2). It is found that the degree of reduction of the NADP(H) pool is always higher than that of the NAD(H) pool (R'NADP(H) greater than R'NAD(H)) except when phi O2 goes to zero (R'NADP(H) approximately equal to R'NAD(H)). Comparison of these results with literature values indicates that the inequality R'NADP(H) greater than R'NAD(H) is always found in a membrane-enclosed compartment, irrespective of the type of transhydrogenase present. This allows an understanding of the function of the two types of pyridine nucleotide transhydrogenases in vivo. The physiological role of non-energy-linked transhydrogenase is to catalyze the reaction NADPH + NAD+ leads to NADP+ + NADH, that of energy-linked transhydrogenase to catalyze the reaction NADH + NADP+ leads to NADPH + NAD+. Since at equilibrium R'NADP(H) approximately equal to R'NAD(H) the inequality R'NADP(H) greater than R'NAD(H) under steady-state conditions explains the energy requirement in the latter reaction. The dependence of the non-energy-linked transhydrogenase activity of ferredoxin:NADP+ oxidoreductase on R'NADP(H) is compared with that of A, vinelandii transhydrogenase. The results indicate that this activity is unlikely to be of physiological importance in plant chloroplasts.