抄録
Effect of nicotinic acid on the formation of catecholamine and the causes of decrease in catecholamine formation has been studied in the nicotinic acid-deficient rats. 1) Concentrations of nicotinic acid in whole brain and liver of rats fed the nicotinic acid-deficient diet were significantly lower than the animals fed the nicotinic acid-supplemented diet. And the levels of NAD and NADH in the brain were significantly lower in the nicotinic acid-deficient rats than those receiving the nicotinic acid-supplemented diet. 2) The activity of tyrosine hydroxylase in whole brain homogenate was increased in the nicotinic acid-deficient rats. No remarkable difference of the enzyme activity in soluble enyzme preparation was observed between the nicotinic acid-deficient and the nicotinic acid-supplemented group. In particulate fractions, however, the specific activity of the enzyme in the nicotinic acid-deficient group was higher than that of the nicotinic acid-supplemented group. When the particle-bound enzyme was solubilized with Triton X-100,no difference of the enzyme activity was observed between the two groups. There was no difference in the Km values for tyrosine between the enzymes purified by ammonium sulfate fractionation of the nicotinic acid-deficient and the nicotinic acid-supplemented groups. Therefore, the increased enzyme activity in the crude enzyme preparation from the nicotinic acid-deficient rat was not due to change of tyrosine hydroxylase, but appeared to be related to endogenous inhibitors. 3) No remarkable difference of the activity of dihydropteridine reductase was observed between the nicotinic acid deficient and the nicotinic acid supplemented group. However, as the concentration of pyridine nucleotide was decreased in the nicotinic acid-deficient rat, it was supposed that the pyridine nucleotide-mediated reduction of 6,7-dihydrobiopterin to the tetrahydro form might not catalyse well. 4) The concentrations of biopterin in the nicotinic acid-deficient rats were significantly lower than the animals fed the nicotinic acid-supplemented diet. The concentrations of biopterin in brain was very smaller than the Km values of tyrosine hydroxylase for biopterin. The studies reported here showed that the decreased formation of catecholamine of the nicotinic acid-deficient rat was due to the decreased concentration of biopterin in brain.
