An Adenosine 3':5'-monophosphate-adenosine Binding Protein from Mouse Liver: Some Physicochemical Properties

A number of physiochemical properties of the cyclic AMP-adenosine binding protein of mouse liver (Ueland, P.M. and Døskeland, S.O. (1977) J. Biol. Chem. 252, 677--686) have been studied. 1. The specific extinction coefficient, E1%280nm, was estimated to 13.0. 2. Amino acid and amide group analyses confirmed the acidic properties of the protein as determined by electrofocusing (pI = 5.7). Based on the estimated partial specific volume (v = 0.74 cm3/g) the minimum molecular weight of the native, tetrameric protein was recalculated to be 185 000 (s20,w = 8.8 . 10(-13) s and Stokes radius = 48 A). 3. No NH2-terminal amino acid was found by the dansyl method using [14C]-dansyl chloride, indicating that the NH2-terminal groups are blocked. 4. Amino acid analyses gave 6 half-cystine residues per subunit, and the same number of free sulfhydryl groups was found by titration of the denatured protein with 5,5'-dithiobis (2-nitrobenzoic acid). 5. The reactivity of the SH groups in the native protein with 5,5'-dithiobis (2-nitrobenzoic acid) revealed rapidly reacting (SHI), sluggishly reacting (SHII) and "masked" (SHIII) SH groups. ATP, adenosine, Mg2+ and KCl, factors known to affect the activation of cyclic AMP binding sites (Ueland, P.M. and Døskeland, S.O. (1978) Arch. Biochem. Biophys., in press) changed the reactivity of separate SH groups.