The metabolism of nitrosopyrrolidine by hepatocytes from Fischer rats

Nitrosopyrrolidine (NO-PYR), an hepatocellular carcinogen, is rapidly metabolized to CO₂ by hepatocytes freshly isolated from the livers of male Fischer rats. Using CO₂ evolution as a measure of NO-PYR metabolism, we observed two kinetic constants; a high affinity component (K_m = 0.11 mM), and a lower affinity component (K_m = 3.2 mM). The high affinity component has similar kinetic constants to those observed for in vitro reactions with microsomes plus cytosol (K_m = 0.36 mM). Therefore, it is probable that the microsomal reaction is the limiting factor in the metabolism of NO-PYR in hepatocytes. NO-PYR may be metabolized to CO₂ through normal anaplerotic sequences. Some metabolites of NO-PYR which have been tentatively identified are γ-hydroxybutyrate, succinic semialdehyde, 3,4-dihydroxybutyric acid lactone, lactate, acetate, pyruvate, glyoxylate, γ-aminobutyrate and alamine. 2-Hydroxytetrahydrofuran (2-hydroxy-THF), a product of α-hydroxylation was detected at low levels in only one of four reactions. 3-Hydroxy-NO-PYR is present but represents only a small percentage of the total metabolism and is probably of little significance in the overall catabolism of NO-PYR in hepatocytes.     

Fatty acid and lactate metabolism by heart homogenates from alloxan-diabetic dogs

Myocardial homogenates from control of alloxan-diabetic dogs were incubated in the presence of lactate or palmitate as the sole substrate. The rate of lactate oxidation of the homogenates from diabetic dogs was less than half of the oxidation rate of homogenates from control animals. The incorporation of labelled palmitate into triglyceride was greatly enhanced in the heart taken from diabetic animals. The tissue concentration of l-carnitine was twice and that of triglyceride was three times as high in the diabetic animals as in their control counterparts. Thus the myocardium of diabetic animals exhibited both altered substrate utilization and increased rate of triglyceride synthesis.     

The metabolism of L-phenylalanine and L-tyrosine by liver cells isolated from adrenalectomized rats and from streptozotocin-diabetic rats.

Flux through, and maximal activities of, key enzymes of phenylalanine and tyrosine degradation were measured in liver cells prepared from adrenalectomized rats and from streptozotocin-diabetic rats. Adrenalectomy decreased the phenylalanine hydroxylase flux/activity ratio; this was restored by steroid treatment in vivo. Changes in the phosphorylation state of the hydroxylase may mediate these effects; there was no significant change in the maximal activity of the hydroxylase. Tyrosine metabolism was enhanced by adrenalectomy; this was not related to any change in maximal activity of the aminotransferase. Steroid treatment increased the maximal activity of the aminotransferase. Both acute (3 days) and chronic (10 days) diabetes were associated with increased metabolism of phenylalanine; insulin treatment in vivo did not reverse these changes. Although elevated hydroxylase protein concentration was a major factor, changes in the enzyme phosphorylation state may contribute to differences in phenylalanine degradation in the acute and chronic diabetic states. Tyrosine metabolism, increased by diabetes, was partially restored to normal by insulin treatment in vivo. These changes can, to a large extent, be interpreted in terms of changes in the maximal activity of the aminotransferase.     

Thyroid hormones and muscle metabolism in dogs

Muscle contents of ATP, ADP, AMP, creatine phosphate and creatine as well as glycogen, some glycolytic intermediates, pyruvate and lactate were compared in the intact, thyroidectomized and triiodothyronine (T3) treated dogs under resting conditions. After thyroidectomy muscle glycogen, glucose 1-phosphate and glucose 6-phosphate contents were significantly elevated while in T3-treated animals these variables were decreased in comparison with control dogs. Muscle free glucose was not altered by thyroidectomy but T3 treatment significantly increased its content. Muscle lactate content was elevated both in hypo- and hyperthyroid animals. Muscle ATP and total adenine nucleotide contents were significantly increased in hyperthyroid dogs while no differences were found between the three groups in the muscle creatine phosphate content. It is assumed that in T3-treated animals carbohydrate catabolism is enhanced in the resting skeletal muscle in spite of high tissue ATP content. Muscle metabolite alterations in hypothyroid dogs seem to reflect the hypometabolism accompanied by a diminished rate of glycogenolysis with inhibited rate of pyruvate oxidation or decreased rate of lactate removal from the cells.     

The metabolism of L-tryptophan by liver cells prepared from adrenalectomized and streptozotocin-diabetic rats.

1. The metabolism of L-tryptophan by liver cells prepared from fed normal, adrenalectomized and streptozotocin-diabetic rats was studied. 2. At physiological concentrations (0.1 mM), the rate of oxidation of tryptophan by tryptophan 2,3-dioxygenase was 3-fold greater in liver cells from diabetic rats than in those from fed rats. In liver cells from diabetic rats, oxidation of tryptophan to CO2 and metabolites of the glutarate pathway was increased 7-fold. Quinolinate synthesis was decreased by 50%. These findings are consistent with an increase in picolinate carboxylase activity. 3. Rates of metabolism of 0.1 mM-tryptophan by hepatocytes from fed and adrenalectomized rats were similar. 4. In all three types of cell preparation, fluxes through tryptophan 2,3-dioxygenase with 2.5 mM-tryptophan were 7-fold greater than those obtained with 0.1 mM-tryptophan. Tryptophan 2,3-dioxygenase and kynureninase fluxes in hepatocytes from fed and adrenalectomized rats were comparable, whereas those in liver cells from diabetic rats were increased 2.5-fold and 3.3-fold respectively. Picolinate carboxylase activities of liver cells from diabetic rats were 15-fold greater than those of cells from fed rats, but rates of quinolinate synthesis were unchanged. 5. It is concluded that: (i) adrenal corticosteroids are not required for the maintenance of basal activities of the kynurenine pathway, whereas (ii) chronic insulin deficiency produces changes in both the rate of oxidation and metabolic fate of tryptophan carbon.