Biphasic changes of tryptophan peroxidase level in tumor-bearing mice and in mice subjected to growth hormone and stress.

It has been well established that the growth of spontaneous, induced, or transplanted tumors in several species is associated with alterations in the enzymatic activity of nontumorous organs. The decrease in liver catalase during tumor growth has been particularly well documented (7). As yet there is no precise understanding of the mecha nisms responsible for these changes, nor of their physiologic and pathologic significance. Therefore, the study of the tryptophan peroxidase-oxidase system (TPO) in liver during the growth of a trans planted neoplasm was undertaken, since its physi ological role (16) and some of the factors regulat ing its level in the tissue are already known. There are at least two independent ways in which the liver TPO level is regulated. It is in creased in vivo by the administration of its sub strate, tryptophan, to intact or adrenalectomized animals, but not by the administration of other compounds (11). This change in TPO activity is similar to the substrate-induced enzyme adapta tions frequently encountered in micro-organisms. The TPO activity is also increased by the adminis tration of cortisone or hydrocortisone to intact or adrenalectomized rats, or by the administration to intact but not adrenalectomized rats of certain compounds which stimulated the adrenal cortex (11, 24). Adrenocorticotrophic hormone, histidine, epinephrine, or histamine increased the TPO ac tivity in this way only in intact rats. Adrenalectomy alone has been shown to lower the liver TPO activity. This new kind of adaptation discovered