The photolysis of complexes between various alkylcobalamins and ethanolamine deaminase was studied by optical and electron spin resonance spectroscopy. In some cases, the products of photolysis of the free and enzyme-bound cobal.imins were characterized. The alkylcobalamins used in these studies were methylcobalamin, β-hydroxyethylcobalamin, δ-(9-adenyl)butylcobalamin, 5′-deoxyinosylcobalamin, and 5′-deoxyadenosylcobalamin. The optical spectra of the enzyme-alkyicobalamin complexes are similar to the spectra of the corresponding free cobalamin derivatives. After photolysis, the spectra of the samples containing enzyme-bound β-hydroxyethyIcobalamin, 5′-deoxyinosylcobalamin, or 5′- deoxyadenosylcobalamin resemble the spectrum of hydroxocobalamin while the spectrum of the enzyme-methylcobalamin complex becomes similar to that of cob(II)alamin and the spectrum of the enzyme-δ-(9-adenyl)butylcobalamin complex is unchanged. The electron paramagnetic resonance spectrum of the unphotolyzed enzyme-5′-deoxyadenosylcobalamin complex displays signals resembling those obtained by a brief anerobic photolysis of 5′-deoxyadenosylcobalamin. Electron paramagnetic resonance spectra of the other unphotolyzed complexes show no signal. After irradiation, the β-hydroxyethylcobalamin, 5′-deoxyinosylcobalamin, and 5′- deoxyadenosylcobalamin complexes show no change, while signals resembling those of anerobically photolyzed 5′-deoxyadenosylcobalamin are exhibited by the methylcobalamin complex. Brief irradiation of the enzyme-δ-(9-adenyl)butylcabalamin complex leads to no change in its electron paramagnetic resonance spectrum, but prolonged irradiation with intense light leads to the appearance of signals similar to those seen in the photolyzed methylcobalamin complex. The major products arising from the organic fragments generated by the photolysis of enzyme-bound methylcobalamin and β-hydroxyethylcobalamin are formaldehyde and acetaldehyde, respectively. However, little of these compounds is produced by the photolysis of the free cobamides. The corrinoid moiety appears to be unaltered by the photolytic process. The implications of these findings are discussed in terms of the possibility that the mechanism of action of coenzyme B12 requiring enzymes might involve a free-radical rearrangement. © 1969, American Chemical Society. All rights reserved.
