VITAMINS

Show abstractHide abstract

The feeding on a diet containing thyroxine to rats significantly increased the conversion ratio of tryptophan to niacin, while, the feeding on a diet containing estrone decreased. The feeding on a diet containing progesterone or testosterone did not affect the conversion ratio. An intraperitoneal injection of adrenaline, alloxan, or streptozotocin caused the decreased conversion ratio. The intraperitoneal injection of prednisolone, a synthetic adrenal cortical hormone, increased the conversion ratio after 1 day ; however, the conversion ratio was lower in the prednisolone group than in control group after 3, 4, 5 and 6 days. On around 7th day, the conversion ratio was restored to the control values. The target of these hormones, by which the conversion ratio was affected, was the activity of liver aminocarboxymuconate-semialdehyde decarboxylase. Thus, the metabolism of tryptophan to niacin is found to be regulated and controlled by the activity of aminocarboxymuconate-semialdehyde decarboxylase via thyroid gland, adrenal, pancreas, and ovary hormones.

View full abstract

Download PDF (869K)

Pyridoxal enzymes catalyze versatile reactions with amino acids, including transamination, recemization, α, β-elimination, and decarboxylation. The reaction mechanism for these reactions has been explained on the basis of diverse chemical properties of pyridoxal phosphate. Recent progress in recombinant DNA techniques allowed us to examine the role of apoproteins of the pyridoxal enzymes at the atomic level based on crystallographic and sitedirected mutagenesis studies. Among the pyridoxal enzymes, aminotransferases, particularly aspartate aminotransferase, have been most extensively studied from structural and functional aspects. In this paper, I will describe some important information obtained recently on substrate recognition and transaldimination, which are the initial steps common to all the pyridoxal enzymes, centering discussion on the uniqueness of aspartate aminotransferase.

View full abstract

Register with J-STAGE for free!