A Separate Antibiotic Binding Site in Xanthosine 5'-Phosphate Aminase: DIFFERENTIAL ALTERATION OF CATALYTIC PROPERTIES AND SENSITIVITY TO INHIBITION

Abstract

Abstract The sensitivity of xanthosine 5'-phosphate aminase to the inhibitory effect of the antibiotic psicofuranine can be reduced by exposure to several kinds of agents capable of modifying protein structure. These include urea and ethylene glycol, reducing agents such as 2-mercaptoethanol, the chelators ethylenediaminetetraacetate and o-phenanthroline, and photo-oxidation with methylene blue. Urea, in causing a 3-fold reduction in the capacity of the aminase to bind psicofuranine and a similar reduction in sensitivity to inhibition by the aminase, also affects other properties of the aminase; the affinity constants for Mg++, NH3, adenosine triphosphate, and xanthosine 5'-phosphate are increased while the activity of the aminase is greatly reduced. 2-Mercaptoethanol reduces the sensitivity of the aminase to inhibition by psicofuranine, but reduces neither its activity nor its ability to bind psicofuranine. In contrast photo-oxidation with methylene blue desensitizes by selectively reducing the ability of the aminase to bind psicofuranine; the substrate-binding capacities of the aminase are not affected by the photo-oxidation. Psicofuranine increases the availability of sulfhydryl groups of the aminase for reaction with sulfhydryl reagents. This indication of a change in the tertiary structure of the aminase together with the additional evidence that the aminase contains a separate binding site for psicofuranine suggests that although the antibiotic is probably bound at a nonessential part of the enzyme it nevertheless may act by distorting the active center.