Histochemical Demonstration of Ascorbic Acid in the Substantia Nigra of the Cat

Abstract

BY the use of histochemical methods, the enzyme tyrosinase has been demonstrated in cells of the substantia nigra in the brain stem of adult cats and monkeys¹. The oxidation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) by the monophenolase activity of tyrosinase is the first step in the synthesis of three aromatic monoamines, dopamine, nor-adrenaline and adrenaline, and also of melanin, which is considered to be the pigment present in the adult human substantia nigra. In the adult, monophenolase activity cannot be demonstrated in nigral cells unless tyrosinase is activated in vitro by some reducing agent, such as ascorbic acid. However, in immature animals—in the cat, from 2 months to 1 year—tyrosinase is active towards tyrosine, and during this period some naturally occurring tyrosinase activator must be present in nigral cells. Ascorbic acid has been found in high concentrations in the adult mammalian brain, notably in certain autonomic centres², and ascorbic acid deficiency leads to interference with tyrosine oxidation³. This vitamin, therefore, may well fulfil the role of activator within the cells of the substantia nigra in immature animals.