DIVISION OF BIOCHEMISTRY: THE ADAPTIVE CONTROL OF TRYPTOPHAN AND TYROSINE METABOLISM IN ANIMALS*

Changes in liver tyrosine aminotransferase after acute and chronic administration of morphine in the rat

Acute administration of Morphine (20 mg/kg/s.c.) in the rat results in a rise of liver tyrosine aminotransferase (TAT) expressed as mumoles of p-hydroxyphenylpyruvate/100 mg/h. With chronic administration, a tolerance develops to this enzymatic effect. TAT induction is not evident in pregnant rats, given the narcotic, in which enzyme levels are already initially high. After delivery TAT returns to normal levels and it is possible to show both induction and tolerance developing to morphine. Enzyme activity in fetal livers is much lower than that of adult animals: after maternal administration of morphine only a modest TAT increase is seen which is not, however, statistically significant. TAT activity is fully evident in livers of offspring, with much higher mean levels in newborn rats from morphine-treated animals, as a possible consequence of morphine deprivation. In this latter group of newborn rats narcotic administration causes TAT activity to return to levels as high as those of naive animals. On the other hand, morphine administration to the prole of naive rats results in an induction of liver TAT.

Tryptophan metabolism in depressive illness

[Methylene-14C]-L-tryptophan was administered intravenously to five patients suffering from severe depression before and after clinical recovery, and to six control subjects; all subjects were tested while on a standard diet. [14C]-5-Hydroxyindoleacetic acid and [14C]-indoleacetic acid were isolated from urine during the following 48 hours and purified to constant specific radioactivity (SA). From the SA and the determination of the total excretion of the two acids their percentage conversion from labelled tryptophan was estimated. There was no consistent difference found, in patients before and after recovery, or between patients and controls, in the conversion of tryptophan to these indole acids. In another series of 11 depressive patients and 11 controls, plasma total (acid-soluble) tryptophan was measured at frequent intervals up to three hours after intravenous administration of a loading dose (1 g) of L-tryptophan. The height of the curves of plasma tryptophan throughout the three hour period was greater than in the depressive patients, even after they had recovered, than in the controls. It is concluded that the apparent volume of distribution of tryptophan is reduced in depressive individuals. This is consistent with other findings of increased protein-binding of plasma tryptophan and reduced CSF tryptophan levels in depressive patients.

Cytoplasmic changes during thioacetamide induced hepatocarcinogenesis in rats

Cytoplasmic changes were investigated in livers of rats at various intervals up to 50 weeks during primary induction of hepatoma by thioacetamide feeding.Microsomal Glucose-6-phosphatase and ATPase activities were shown to decrease progressively with increased period of thioacetamide feeding the fall in activities being more pronounced during the first 15 weeks.Hormonal induction of tryptophan pyrrolase and tyrosine transaminase activities was shown to undergo a significant decrease of 65% and 55% respectively at the end of 50 weeks feeding.The substrate induced tryptophan pyrrolase activity was decreased to 50% during the 50 weeks period whereas the substrate induced tyrosine transaminase activity gradually increased to 200%. The latter is attributable to differences in the optimal induction dose of tyrosine in normal and carcinogen fed rats.The m-RNA template lifetime for tryptophan pyrrolase was shown to be exceeding 24 hours in normal rats as against that of 13 hours in rats fed with carcinogen for 30 weeks. On the other hand the m-RNA template lifetime for tyrosine transaminase was 3 hours in control rats while it was 7 hours in the carcinogen fed rats.The observed changes were shown to occur long before the onset of malignant transformation.The alterations in terms of decreased Glucose-6-phosphatase and substrate induced tryptophan pyrrolase activities were shown to be reversible when the carcinogen was withdrawn from the diet after 30 weeks of feeding.The significance of these observations is discussed in relation to damage to endoplasmic reticulum during hepatocarcinogenesis.

Maturation of a stress-activated mechanism inhibiting induction of tyrosine transaminase

Rats of various ages were subjected to the stress of 30 minutes on a noisy reciprocating shaker 4 hours before their liver tyrosine transaminase and tryptophan pyrrolase activities were measured. Adrenalectomized infants and adults and hypophysectomized adults were also stressed. Intact, stressed infants exhibited an increase in tyrosine transaminase activity, while intact, stressed adults showed no change. In the stressed adrenalectomized adult, tyrosine transaminase activity markedly decreased, while adrenalectomized infants showed no change. Hypophysectomy largely, but not completely, abolished inhibition in the adults. Tryptophan pyrrolase activity, when present, was increased by stress in all age groups, but the increase was abolished by adrenalectomy and hypophysectomy. The results suggest stress-activation of a pituitary mechanism that inhibits or represses activation of tyrosine transaminase and that may not function during early postnatal life.