Effects of irradiation and starvation on the regulation of rat liver enzymes

Time-dependence of the effect of X-irradiation on the formation of glycerol phosphate dehydrogenase and other dehydrogenases in the developing rat brain

X-irradiation (100-1500 r) administered to the heads of rats 8-30 days of age inhibited the development of glycerol phosphate dehydrogenase (l-glycerol 3-phosphate-NAD oxidoreductase, EC 1.1.1.8) in the brain stem and cerebral hemispheres. At 40 days of age and older no effect was observed. This inhibition was a delayed phenomenon, dose-dependent and with no recovery. It is proposed that the inhibition of enzyme formation is related to radiation damage caused to DNA. Actinomycin D inhibited the development of glycerol phosphate dehydrogenase in a manner similar to ionizing radiation. Four other dehydrogenases also showed age-dependent radiosensitivities. ;Malic enzyme' (EC 1.1.1.40), lactate dehydrogenase (EC 1.1.1.27) and malate dehydrogenase (EC 1.1.1.37) ceased to be radiosensitive at about 8 days of age and isocitrate dehydrogenase (NADP) (EC 1.1.1.42) at 16 days. The correlation between developmental increase in enzyme activity and radiosensitivity held closely for glycerol phosphate dehydrogenase and isocitrate dehydrogenase and to a smaller extent for the others.

Circadian rhythms of liver enzymes and their relationship to enzyme induction

Tyrosine alpha ketoglutarate transaminase, which is rapidly induced by various agents, shows circadian rhythmicity in the intact rat. This rhythmicity is only slightly altered after adrenalectomy, indicating that adrenal hormones do not play a major role in the metabolic control of the activity of tyrosine alpha ketoglutarate transaminase. On the other hand, phenylalaninepyruvate transaminase, which is not inducible over the same time period, does not show circadian variation. The results suggest that the sensitivity of an enzyme's regulating system to inducing agents may be related to the inherent circadian rhythm of the enzyme.

Inhibition of lipolytic action of growth hormone and glucocorticoid by ultraviolet and x-radiation

Addition of catecholamines or growth hormone plus glucocorticoid activates lipolysis in isolated white-fat cells of the rat. If the fat cells were irradiated just before the addition of growth hormone and glucocorticoid, their lipolytic action was abolished. However, irradiation did not affect the activation of lipolysis by catecholamines.