CHEMICAL AND BIOCHEMICAL STUDIES ON VITAMIN B12 AND ITS RELATED COMPOUNDS:XVI. FACTORS AFFECTING THE ACTIVITY OF A DIOLDEHYDRASE REQUIRING COBAMIDE COENZYME. ENHANCEMENT BY ANAEROBIC LIGHT-IRRADIATION AND COMPETITIVE INHIBITION BY ADENOSINE

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1. Influences of light-irradiation on the enzymic conversion of propanediol to propionaldehyde catalyzed by 5, 6-dimethylbenzimidazolyl cobamide coenzyme-dependent dioldehydrase were examined under the following conditions: (a) holoenzyme was irradiated, prior to the incubation with the substrate in the dark, under aerobic and anaerobic conditions, respectively; (b) light was irradiated on the mixture of apoenzyme and the coenzyme at 0° where the holoenzyme was hardly formed, then incubated with the substrate in the dark; (c) illumination was carried out after the enzymic reaction began to proceed steadily.2. The irradiation on the holoenzyme under anaerobic conditions stimulated the reaction rate twice that without illumination. On the other hand, the enzyme activity decreased to less than a half under aerobic condition. The accelerating effect of the illumination under anaerobic conditions is considered to be due to the anaerobic cleavage of the cobalt-carbon bond in the enzyme linking a 5′-deoxyadenosyl moiety, which forms a protein-bound B12r. and a 5′-deoxyadenosyl radical, even if the protein-bound B12r may be further reduced to the state of B12s before the reaction with the substrate. The ineffectiveness of the aerobic illumination indicates that hydroxocobalamin and adenosine-5′-aldehyde or adenosine-5′-carboxylic acid bound to the apoprotein cannot react with the substrate. Under the condition b, in which the coenzyme was in part transformed to hydroxocobalamin by irradiation, the coenzyme activity decreased to the same extent as that of the aerobic photolysis of the holoenzyme mentioned above. When the enzymic reaction began to proceed steadily (condition c), light did not affect the reaction rate.3. Associated with the appearance of a B12r-like spectrum in the course of the enzymic reaction, the enhancement of the holoenzyme by anaerobic photolysis would be attributed to the stimulation of the cleavage of the cobalt-carbon bond followed by the formation of a protein-bound B12r and 5′-deoxyadenosyl radical, at least as an intermediary step.4. The ineffectiveness of irradiation on the incubation mixture of the apoenzyme with methylcobalamin, which is known to be a competitive inhibitor in the enzyme system, suggests that the enzymic reaction cannot be mediated by a protein-bound B12r or B12s alone but that protein-bound 5′-deoxyadenosyl part is also necessary for the reaction. The possible participation of 5′-deoxyadenosyl moiety was supported by the competitive inhibition caused by the addition of adenosine.
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CHEMICAL AND BIOCHEMICAL STUDIES ON VITAMIN B12 AND ITS RELATED COMPOUNDS:XVI. FACTORS AFFECTING THE ACTIVITY OF A DIOLDEHYDRASE REQUIRING COBAMIDE COENZYME. ENHANCEMENT BY ANAEROBIC LIGHT-IRRADIATION AND COMPETITIVE INHIBITION BY ADENOSINE

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