Biochemistry of Amino Acids - A. Meister 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Amino Group Metabolism
Hormonal Influences

Nitrogen METABOLISM in mammals is under the direct or indirect control of hormonal systems. The existence of a regulatory mechanism is evidenced by the fact that, under normal conditions, the mammalian Organism maintains a state of nitrogen equilibrium. The ability of certain Hormones to alter the overall state of nitrogen metabolism is well established. For instance, it has been found that Insulin deficiency leads to a negative nitrogen balance, whereas the administration of certain Other Hormones (particularly androgens) results in nitrogen retention.

Although numerous observations of this kind have been described in the literature, the mechanisms through which hormones influence nitrogen metabolism remain unknown. The Effect of certain hormones on the free amino acid content in the Blood Plasma of experimental animals has been studied. It has been found that insulin, Growth Hormone, and adrenaline decrease plasma amino acid levels [88—92]. According to Griffin and co-workers [93], adrenaline appears to be one of the final links in the chain of agents responsible for this effect. Indeed, growth hormone and insulin fail to affect plasma free amino acid levels in adrenalectomized animals, whereas adrenaline lowers blood amino acid levels in both hypophysectomized and adrenalectomized animals. It is also noteworthy that pituitary thyrotropic hormone decreases blood amino acid content in normal, adrenalectomized, and hypophysectomized rats alike.

There is a considerable body of observations concerning Other Aspects of hormonal influence on nitrogen metabolism; it is now evident that investigating these hormonal effects constitutes an exceptionally complex problem. Russell [94], in one of the recent reviews on the subject, arrives at the following Conclusion:

“At present, we can perhaps state that of the three major hormones playing an important role in this regard [in The regulation of nitrogen metabolism]—namely, insulin, Adrenocortical Hormones, and growth hormone—none exerts an effect directed primarily or exclusively on Amino Acid Catabolism as such, or on Gluconeogenesis from amino acid fragments; rather, all these hormones evidently influence various aspects of Protein Synthesis or degradation in most or many Tissues. However, The Mechanism of their regulatory action remains completely unknown.”

Studies focusing on the effect of hormones on the Metabolism of individual Amino Acids are relatively scarce. The Influence of hormonal agents on the Tryptophan peroxidase system [95, 96] possibly represents one of those instances where the mechanism governing the synthesis of a specific enzyme exerts a targeted effect on the catabolic rate of a particular amino acid.