GLUCOCORTICOID STIMULATION OF THE BIOSYNTHESIS OF GLUTAMIC-ALANINE TRANSAMINASE

Abstract

Results.-In order to interpret meaningfully the results of measurements of the influence of hormones on amino acid incorporation into a specific protein, it is necessary to know that the period of exposure to the labeled amino acid is short relative to the half-life of the enzyme. Otherwise, the enzyme may have become fully labeled during the experimental period under all conditions. This information is lacking in the studies with carbamyl phosphate synthetase. The half-life of a protein can be measured by isotopic methods if it can be isolated, or more simply by enzyme activity measurements, if a method is available for perturbing its steadystate level in tissues. The latter possibility is readily realized with enzymes, the activity levels of which in tissues are susceptible to hormonal alteration, once it has been established that the activity level is a valid measure of the amount of catalytically active protein present, as in the case at hand.7 The prednisolone-mediated transformation from the normal steady-state level to what may be called the high glucocorticoid steady-state level is shown in Figure 1. The high hormone steady-state level is virtually achieved in 5 days, as previously reported by Rosen et al.1 On the eighth day, at the point marked by arrows on the figure, hormone administration was discontinued, and the activity allowed to return 912