Schwartz Phillip A, Frey Perry A
Department of Biochemistry, University of Wisconsin-Madison, 1710 University Avenue, Madison, Wisconsin 53726, USA.
Biochemistry. 2007 Jun 19;46(24):7284-92. doi: 10.1021/bi700077v. Epub 2007 May 16.
The photolysis of adenosylcobalamin (coenzyme B12) results in homolytic cleavage of the Co-C5' bond, forming cob(II)alamin and the 5'-deoxyadenosyl radical. In the presence of molecular oxygen, it has been proposed that the primary reaction is interception of the 5'-deoxyadenosyl radical by O2 to form adenosine-5'-aldehyde as the product (Hogenkamp, H. P. C., Ladd, J. N., and Barker, H. A. (1962) J. Biol. Chem. 237, 1950-1952). 5'-Peroxyadenosine is here found to be the initial nucleoside product of this reaction and found to decompose to adenosine-5'-aldehyde. Evidence indicates that 5'-peroxyadenosine arises from the hydrolysis of 5'-peroxyadenosylcobalamin, with the formation of cob(III)alamin. 5'-Peroxyadenosine undergoes further decomposition to adenosine-5'-aldehyde as the major final product of aerobic photolysis as well as to adenosine and adenine as minor products. In a cobalamin-dependent process, 5'-peroxyadenosine becomes re-ligated to cob(III)alamin to form 5'-peroxyadenosylcobalamin, which quickly decomposes to adenosine-5'-aldehyde and cob(III)alamin. This is supported by spectrophotometric observations of both rapidly photolyzed adenosylcobalamin and the reaction of cob(III)alamin with excess 5'-peroxyadenosine. 5'-Peroxyadenosine also slowly undergoes cobalamin-independent decomposition to adenosine-5'-aldehyde and the minor products adenosine and adenine. The present study provides a detailed description of the products initially formed when aqueous, homolytically cleaved adenosylcobalamin reacts with molecular oxygen and provides a detailed description of the behavior of those products subsequent to photolysis.
腺苷钴胺素（辅酶B12）的光解作用导致Co-C5'键的均裂，形成钴胺素（II）和5'-脱氧腺苷自由基。在有分子氧存在的情况下，有人提出主要反应是O2截获5'-脱氧腺苷自由基，形成腺苷-5'-醛作为产物（霍根坎普，H.P.C.，拉德，J.N.，和巴克，H.A.（1962年）《生物化学杂志》237卷，1950 - 1952页）。在此发现5'-过氧腺苷是该反应的初始核苷产物，并发现其分解为腺苷-5'-醛。有证据表明5'-过氧腺苷源于5'-过氧腺苷钴胺素的水解，同时形成钴胺素（III）。5'-过氧腺苷会进一步分解为腺苷-5'-醛，这是需氧光解的主要最终产物，还会分解为腺苷和腺嘌呤等次要产物。在一个依赖钴胺素的过程中，5'-过氧腺苷重新与钴胺素（III）连接形成5'-过氧腺苷钴胺素，后者会迅速分解为腺苷-5'-醛和钴胺素（III）。这一点得到了对快速光解的腺苷钴胺素以及钴胺素（III）与过量5'-过氧腺苷反应的分光光度观测结果的支持。此外，5'-过氧腺苷也会缓慢地发生不依赖钴胺素的分解，生成腺苷-5'-醛以及次要产物腺苷和腺嘌呤。本研究详细描述了均裂后的水溶液中腺苷钴胺素与分子氧反应时最初形成的产物，并详细描述了光解后这些产物的行为。
