Adenosine deaminase from calf intestine hydrolyzes adenine at a limiting rate four orders of magnitude lower than that for adenosine, while Km values for these substrates are about the same (Wolfenden, R., et al. (1969), Biochemistry 8, 2412-2415). Reactivity of 6-substituents, toward nucleophilic displacement, is found to be affected only slightly by removal of ribose as a 9-substituent, in model reactions. Substituent ribose thus appears to stabilize, selectively, the transition state for enzymatic deamination. In contrast with the small influence of substituent ribose on the apparent binding affinity of substrates, removal of substituent ribose from a potential transition state analogue, 1,6-dihydro-6-hydroxy-methylpurine ribonucleoside, results in a lowering of its affinity for the enzyme by several orders of magnitude. The synthesis of the analogue and related compounds is described, and their properties compared with those of other photoadducts and of the naturally occurring inhibitors covidarabine and coformycin. Binding of these inhibitors is found to result in the appearance of ultraviolet-absorbing bands in the neighborhood of 323 nm.

Cytidine deaminase from Escherichia coli was found to catalyze the hydrolytic deamination of 5,6-dihydrocytidine, at a rate slightly lower than its rate of action on the normal substrate. The results suggest that nucleophilic addition by the enzyme at the 5,6 position of the substrate is not an essential part of catalysis, unless the active site is so flexible that deamination can occur with addition in one case (cytidine) and without addition in another case (5,6-dihydrocytidine). 3,4,5,6-Tetrahydrouridine bears a close structural resemblance to a hypothetical "tetrahedral" intermediate formed by direct water addition to 5,6-dihydrocytidine. The hydrolytic activity of the enzyme toward 5,6-dihydrocytidine and its potent inhibition by 3,4,5,6-tetrahydrouridine are presumably related by the ability of the active site to stabilize structures of this kind by tight binding. Cytidine deaminase shows no detectable activity as a catalyst for the dehydration of 6-hydroxy-5,6-dihydrouridine.

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oped with petroleum ether (bp 30-6O0)-ether (2:1)] to give two fractions in addition to additional pure 2d (total recovery 52%).The first of these was a mixture (nmr)3 of syn alcohol Id (-10% yield by nmr) and anti formate 2g (30% yield) which have nearly identical Rf values. The anti formate, which was unknown, was purified by recrystallization of this mixture from chloroformpetroleum ether (bp 60-90"): nmr (CDC13) 6 8.19 (1, s, 02CH), 8.19-7.58 (4, m, aromatic H), 6.23 (1, X portion of ABX, JAX + J B X = 16 Hz, q, CHOzCH), 3.75-0 (broad m, 18, bridge CH2).Anal. Calcd for C20H24NOzC1: C, 69.45; H, 6.99; N, 4.05.Found: C, 69.68; H, 7.01; N, 4.04. The mother liquor from this recrystallization contained (tlc and nmr) syn alcohol ld, anti formate Id, and trace amounts of another material assumed to be syn formatedg,The other fraction (5.5 mg, 6% yie1d)was reduced pyridinophane IC (mp and mmp4b of picrate 173-174').Thermal Stability of Syn and Anti Bromides (lb and 2b). A.A solution of syn l b (50.5 mg, 0.133 mmol) in xylene (ACS grade, 0.5 ml) was heated at the reflux temperature under nitrogen for 220 hr. Xylene was removed (in vacuo) from the brown, tarry mixture; the residue showed (nmr) unchanged 2a (6 6.06) and no epimer 2b (6 5.42). The sample was dissolved in chloroform and washed with dilute sodium bicarbonate and the product was recrystallized from petroleum ether to give 41.6 mg (82.3% recovery) of 2a, m p 141-142", mmp 143-144" with material melting at B. A sample of anti 2b (65.8 mg, 0.173 mmol) was treated in xylene as described above. The nmr spectrum of the crude product showed no 2b (6 5.42) or 2a (6 6.06) and was quite similar to that of IC. The product was decolorized as above and purified by preparative tlc [silica gel, petroleum ether-ether (3:1)] to give 26 mg (50% yield) of IC (mp and mmp 78-79"). ., 35,3775 (1970).Aikylsuifonates normally undergo carbon-oxygen scission under these conditions: the authors are unaware of related acid-catalyzed S-0 bond cleavage. 2-and 4-methyipyridines undergo hydrogen-deuterium interchange on the methyl group, presumably through a carbanion intermediate, under rather mild conditions: cf. K. Schofieid, "Hetero-Aromatic Nitrogen Compounds," Butterworths, London, 1967, pp 324-327, and references cited therein. Sulfur-oxygen bond cleavage of aikylsuifonates by base is uncommon, since substitution or elimination at carbon generally occurs. However, aryisuifonates readily undergo S-0 bond cleavage by nucleophiles: cf. W. D. Ciosson and P. Wriede, J. Amer. Chem. Soc., 88,1581 (1966).

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