Transport systems for tyrosine and their relation to catecholamine biosynthesis

The transport of tyrosine into nerve tissue has been scarcely studied. The main reason for this lack of interest may be the assumption that the intraneuronal concentration of tyrosine is sufficient to saturate the enzyme tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of catecholamines. Recently, however, evidence has been presented that the extraneuronal concentration of tyrosine may act as one of the factors controlling catecholamine biosynthesis in the absence of feed-back repression of tyrosine hydroxylase (Wurtman et al., 1974; Gibson and Wurtman, 1977). These results suggest that the intraneuronal concentration is not high enough to saturate tyrosine hydroxylase. Since the feed-back repression of tyrosine hydroxylase is discontinued during depolarization of catecholaminergic nerve fibres, as a result of catecholamine release (see Weiner, 1970), it is not inconceivable that during depolarization an enhanced tyrosine transport into the nerve ending will contribute to the increased biosynthesis. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.