Effect of inhibitors on the activity of soluble succinic dehydrogenase and on the reconstitution of the succinic dehydrogenase-cytochrome system from its components

David Keilin, 1887-1963

The sudden death on 27 February 1963 of David Keilin deprived the scientific world of a biologist the span of whose scientific activities is unlikely to be equalled, much less excelled, in the future. These activities extended from descriptive morphology of protists, fungi and insects to the biochemistry of respiratory enzymes and metallo-protein compounds. It is, consequently, not possible for one person to deal adequately with all aspects of his work and in this appreciation of the man and his scientific achievements the main emphasis will be on his biological work. That bearing on parasitology will be considered in some detail, whereas the later, more biochemical, work will be considered briefly as fuller accounts of it have already been given by E. F. Hartree (Biochem. J.89, 1–5), T. R. R. Mann (Biogr. Mem. Fellows R. Soc.10, 185–207) and E. C. Slater (Enzymologia, 26, 313–20). In the brief survey of his biochemical work I have made much use of summaries and reports written by Keilin himself and I have often given the essence of lines of work summed up in his own words which express his conclusions with the elegance, lucidity and brevity characteristic of his style. As I was closely associated with David Keilin and in almost daily contact with him for just on forty years, it is natural that the following account will to some extent reflect my own interests and those aspects of his personality which became manifested during long and intimate contact both in private life and collaboration in scientific and administrative work.

Absorption spectra and some other properties of cytochrome c and of its compounds with ligands

A brief account is given of the development of our knowledge of certain aspects of cytochrome c since its early preparations from baker’s yeast, on which the fundamental properties of this haemoprotein were established. In view of the extensive work which is now being carried out on cytochromes of group c isolated from different organisms, it was found impor­tant to re-investigate certain properties of cytochrome c which can easily be prepared from mammalian heart muscle and purified to 0.45 % Fe. The absorption spectra of oxidized (Fe 3+ ) and reduced (Fe 2+ ) cytochrome c (at pH 6.8, 13 and 14) and of their compounds with ligands such as CO (Fe 2+ ), CN' (Fe 2+ and Fe 3+ ), N' 3 (Fe 3+ ), NO (Fe 2+ and Fe 3+ ) and F'(Fe 3+ ) were re-determined and uniformly presented. Marked intensification of the Soret- and α-bands of ferro-cytochrome c was observed at pH14. Although CN'- and N' 3 -ferri-cytochrome c are formed within the physiological range of pH, they play no part in cyanide or azide inhibition of cell respiration or of the catalytic activity of the respiratory chain in the heart muscle preparation. Nitric oxide reacts with both ferri- and ferro-cytochrome c forming two spectroscopically distinct compounds: I and II analogous to compounds of NO with methaemoglobin and haemoglobin respectively. The nature of these compounds is discussed in relation to the view recently put forward that compound I, which was found to be diamagnetic, has a ferro-cytochrome c nitrosyl structure. The inability of ferro-cytochrome c to react with O 2 and ligands (CO, CN ' and NO) within the physiological range of pH, is due not only to the embedded position of the haem within the fold of the protein, to which it is linked both by its iron and porphyrin side chains (2 and 4), but also to the strong affinity of the divalent iron for the nitrogenous groups of the protein, as well as to the nature of the protein fold which exerts greater steric hindrance to ligands in ferro- than in ferri-cytochrome c . This is in agreement with the view that the tertiary protein structure is more open in ferri- than in ferro-cytochrome c . Criteria of the purity of cytochrome c were re-examined in the light of recent work on its purification. The thermostability of cytochrome c recognized during the early study of this compound was further investigated. It was found that ferro-cytochrome c in solutions at pH 9.1 to 10.2, containing carbon monoxide or cyanide, when heated to 100 °C becomes completely con­verted into the corresponding CO- or CN'-derivative, which, on cooling, immediately dissociate; up to 80% of heat-denatured cytochrome c was found to revert to the original native state. The striking similarities of yeast and mammalian cytochrome c in all their properties, including their equally successful incorporation within the cytochrome c deficient heart muscle succinic oxidase system, contrasts with the marked dissimilarities in the structures of their haemopeptide cores, which were found to differ in the contents and sequences of five of their eleven amino acids. Similar discrepancies between the primary protein structures and the biological properties have recently been recorded for the α and β peptide chains of human haemoglobin and the single chain of sperm whale myoglobin. The effect of the valency of iron on the general properties of protein of cytochrome c and of other haemoproteins is discussed.

FLAVINE ADENINE DINUCLEOTIDE-LINKED MALIC DEHYDROGENASE FROM ACETOBACTER XYLINUM

Benziman, Moshe (The Hebrew University of Jerusalem, Jerusalem, Israel), and Y. Galanter. Flavine adenine dinucleotide-linked malic dehydrogenase from Acetobacter xylinum. J. Bacteriol. 88:1010-1018. 1964.-The properties of the pyridine nucleotide-nonlinked malic dehydrogenase of Acetobacter xylinum were investigated in the supernatant fluid obtained by high-speed centrifugation of sonic extracts. Ferricyanide, phenazine methosulfate, and to a lesser extent dichlorophenolindophenol were active as oxidants for malate oxidation. After acid ammonium sulfate precipitation, the enzyme lost its malate-oxidizing activity. The enzyme was reactivated by low concentrations of flavine adenine dinucleotide (FAD) but not by flavine mononucleotide (FMN) or riboflavine. Atabrine inhibited the enzyme, and the inhibition was relieved by FAD but not by FMN or riboflavine. Malate-oxidizing activity was inhibited by hematin. The inhibition was prevented by imidazole or globin. o-Phenanthroline, 8-hydroxy quinoline, alpha,alpha'-dipyridyl, and p-chloromercuribenzoate inhibited malate oxidation. Amytal markedly inhibited oxidation of malate in the presence of oxygen, phenazine methosulfate, or dichlorophenolindophenol, but not in the presence of ferricyanide. The results suggest that the malic dehydrogenase of A. xylinum is a FAD enzyme, which contains an ironbinding site essential for its activity. Nonheme iron and sulfhydro groups are possibly involved in enzyme activity. The malic dehydrogenase is functionally linked to the cytochrome chain.

Reactivity of the sulfhydryl groups of soluble succinate dehydrogenase

Soluble succinate dehydrogenase prepared by butanol extraction reacts with N-ethylmaleimide according to first-order kinetics with respect to both remaining active enzyme and the inhibitor concentration. Binding of the sulfhydryl groups of the enzyme prevents its alkylation by N-ethylmaleimide and inhibition by oxaloacetate. A kinetic analysis of the inactivation of alkylating reagent in the presence of succinate or malonate suggests that N-ethylmaleimide acts as a site-directed inhibitor. The apparent first-order rate constant of alkylation increases between pH 5.8 and 7.8 indicating a pKa value for the enzyme sulfhydryl group equal to 7.0 at 22 degrees C in 50 mM Tris-sufate buffer. Certain anions (phosphate, citrate, maleate and acetate) decrease the reactivity of the enzyme towards the alkylating reagent. Succinate/phenazine methosulfate reductase activity measured in the presence of a saturating concentration of succinate shows the same pH-dependence as the alkylation rate by N-ethylmaleimide. The mechanism of the first step of succinate oxidation, including a nucleophilic attack of substrate by the active-site sulfhydryl group, is discussed.