46) NOTE ADDEO IN PROOF, The relatively higher stability of Nmethylimidazole vs. imidazole complexes suggested in our preliminary communication (ref 23) has not been substantiated by further work. The present evidence suggests that both heterocycles have about the same affinity for magnesium in its complexes. In complexes of histidine and Nr-methylhistidine with magnesium phosphates in the enzyme active site pocket, the unmethylated histidine could give rise to a tighter binding than the Kmethyl derivative for purely steric reasons.
Abstract:The photolysis o f prednisone acetate ( l b ) in dioxane yielded lumiprednisone acetate (Zb) and not the previously proposed structure 3. Further photoisomerization o f 2b in dioxane gave 2 1 -acetoxy-2, I7a-dihydroxy-I m e t h y l -19-norpregna-1,3,5( IO)-triene-I I ,20-dione (lob). Irradiation o f 2b in 45% aqueous acetic acid yielded 2 I -acetoxy-l I a, I7a-dihydroxy-Ip.1 Ifi-oxa-IOa-pregna-2,20-dione (14a), which has added a molecule o f water. Similar results were observed when prednisone was irradiated. Treatment o f 2b with acid afforded 17a,21 -dihydroxy-I ( I 0 -SP)-abeo-pregna-I ,9-diene-3, I 1,20-trione (15). The mechanism o f these photoisomerization reactions is discussed. The influence o f solvents and the I I -keto function on the photochemistry of the bicyclo[3. I .O]hex-3-en-2-one system is explained.
ResultsA. Irradiations in Neutral Media. Irradiation of l a or l b in dry dioxane with 254-nm light afforded the "lumiprednisones" 2a and 2b, respectively, in 65% yield. The assignment of the structure and stereochemistry of 2a and 2b was by comparison of their spectral data with those of the lumiproducts, 9a and 9b, derived by photoisomerization of I7P-acetoxy-1 -dehydrotestosterones 8a4 and 8b,5 respectively, in dioxane with 254-nm light. The structure of the lumiproduct 9a was proven by chemical degradation4 and by circular dichroism mea-suremenk6 The UV of the lumiproduct 2b, A , , , 266 nm ( E 2500), is in close agreement with that of 9b, 268 nm ( e 2950).5The 'H NMR spectra of the A-ring protons in the lumipro-Abstract: Acid and alkaline phosphatase reacts with phosphoenolpyruvate (PEP) to generate enolpyruvate, but neither ketopyruvate nor the geminate diol of pyruvate. The adsorption spectrum for the phosphatase product was derived by kinetic correlation of the changing spectra. Its A, , , , (225 nm) and molar absorptivity, €225 9600 M-I cm-I, are appropriate for 2-hydroxyacrylic acid. Mass spectral analysis shows that the phosphatase product that accumulates transiently requires the addition of a proton to C-3 to give pyruvate. The protonation is slowed ca. sixfold in DrO compared to HzO. Enolpyruvate has sufficient stability ((112 = 3.6 min at 20 OC in DzO, pD 6.4) to be examined as a possible intermediate in enzymatic catalysis. It was predictably shown to lead to strong inhibition of lactate dehydrogenase in the presence of DPN+. Catalysis by pyruvate kinase of the ketonization of enolpyruvate, generated with phosphatase in situ, was shown to occu...
