Glucocorticoid Stimulation of the Biosynthesis of Glutamic-alanine Transaminase.

Among the mechanisms which have been considered as possible descriptions of the molecular events underlying hormone action, one which has attracted increasing attention among contemporary biochemists is the proposal that control of the biosynthesis of specific enzymes may be an important factor (see, e.g., ref. 1). The plethora of enzyme activities which are found to increase or decrease in response to an alteration in endocrine state in the whole animal2 has been an insistent factor in favor of this view. 'I\ore sophisticated studies have come forth in recent years which have given further encouragement to this concept. These have been of three types, of which the following are examples. It has been demonstrated that there is an increased amino acid incorporation into carbamyl phosphate synthetase of tadpole liver after exposure of the animals to thyroxine3 and into a partially purified preparation of glutamic-tyrosine transaminase of rat liver after exposure of the animals to cortisol.' Early hormonal effects have been found on RNA metabolism,5' 6 particularly of the nuclear fraction.7' 8 Finally, altered hormonal states have a striking influence on amino acid incorporation into total protein in cell-free systems.'-" We are presenting in this paper what we believe to be unequivocal evidence that a change in endocrine state leads to a change in the rate of biosynthesis of a specific enzyme. Materials and Methods.-The definition of activity units and methods of enzyme assay and antibody titration of rat liver glutamic-alanine transaminase have been presented previously."6 17 The procedure for purification of the enzyme to a homogeneous state and some of its physical and chemical properties will be published elsewhere. 18 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.3 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.19 On the eighth day, at the point marked by arrows on the figure, hormone administration was discontinued, and the activity allowed to return