Why Are Two Different Types of Pyridine Nucleotide Transhydrogenase Found in Living Organisms?

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Abstract
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 in found in certain heterotrophic bacteria. The presence of a structurally similar non-energy-linked transhydrogenase in Azotobacter vinelandii, Pseudomonas aeruginosa and P. 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 [spinach] 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 2 types of pyridine nucleotide transhydrogenases, the steady-state degrees of reduction of the NADP(H) and NAD(H) pools in A. vinelandii (R''NAD(H) and R''NAD(H)) were measured for cells metabolizing sucrose at a variable oxygen flux (.vphi.O2). The degree of reduction of the NADP(H) pool is always higher than that of the NAD(H) pool (R''NADP(H) > R''NAD(H)) except when .vphi.O2 goes to zero (R''NADP(H) .apprxeq. R''NAD(H)). Comparison of these results with literature values indicates that the inequality R''NADP(H) > 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 2 types of pyridine nucleotide transhydrogenases in vivo. The physiological role of non-energy-linked transhydrogenase is to catalyze the reaction NADPH + NAD+ .fwdarw. NADP+ + NADH, that of energy-linked transhydrogenase to catalyze the reaction NADH + NADP+ .fwdarw. NADPH + NAD+. Since at equilibrium R''NADP(H) .apprxeq. R''NAD(H) the inequality R''NADP(H) > 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 importane in plant chloroplasts.