title: Spontaneous epimerization of (S)-deoxycoformycin and interaction of (R)-deoxycoformycin, (S)-deoxycoformycin, and 8-ketodeoxycoformycin with adenosine deaminase.
abstract: (R)-Deoxycoformycin (pentostatin), (S)-deoxycoformycin, and 8-ketodeoxycoformycin were compared as inhibitors of calf intestine adenosine deaminase. In contrast to (R)-deoxycoformycin, which had been demonstrated as a tight-binding inhibitor with a dissociation constant of 2.5 X 10(-12) M [Agarwal, R. P., Spector, T., & Parks, R. E., Jr. (1977) Biochem. Pharmacol. 26, 359-367], (S)-deoxycoformycin and 8-ketodeoxycoformycin are slope-linear competitive inhibitors with respect to adenosine. The kinetic constants are 33 microM for inhibition by (S)-deoxycoformycin, 43 microM for 8-ketodeoxycoformycin, and 16 microM for the Km for adenosine. The stereochemistry of carbon 8 of the diazepine ring therefore causes a (1.3 X 10(7]-fold change in the affinity for the enzyme which is specific for the R configuration. This difference is attributed to an induced conformational change which cannot be initiated by the S isomer or the 8-keto analogue of (R)-deoxycoformycin. The studies were complicated by the need to remove traces of tight-binding inhibitor(s) from (S)-deoxycoformycin, since as little as 0.001% of the R isomer causes significant inhibition. The R and S isomers of deoxycoformycin are unstable in neutral or mildly acidic aqueous solutions. Isomerization of the secondary hydroxyl at carbon 8 of the diazepine ring is one of the reactions, resulting in S to R and R to S conversions for deoxycoformycins. Opening of the aglycon is also a major reaction. The tight-binding inhibitor generated from (S)-deoxycoformycin was identified as (R)-deoxycoformycin by high-pressure liquid chromatography, spectroscopy, circular dichroism, and chemical criteria.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000243:Adenosine Deaminase / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D002942:Circular Dichroism; D003070:Coformycin / Q000031:analogs & derivatives / Q000302:isolation & purification / Q000494:pharmacology*; D007422:Intestines / Q000201:enzymology; D007536:Isomerism; D007700:Kinetics; D008968:Molecular Conformation; D009700:Nucleoside Deaminases / Q000378:metabolism*; D015649:Pentostatin / Q000031:analogs & derivatives; D012263:Ribonucleosides / Q000494:pharmacology*
chemical_list: D012263:Ribonucleosides; D003070:Coformycin; D015649:Pentostatin; C045284:8-ketodeoxycoformycin; D009700:Nucleoside Deaminases; D000243:Adenosine Deaminase