Protein-bound dopa and 5-S-cysteinyldopa in non-melanogenic tissues

To explore the possibility that dopa and 5-S-cysteinyldopa, precursors of melanin, can be produced in non-melanogenic tissues, this study examined the contents of the free and protein-bound forms of two catechols in non-melanogenic tissues of mice and rats, and compared the urinary excretion of free catechols in black and albino mice. Considerable amounts of protein bound dopa and 5-S-cysteinyldopa were present in the hair of tyrosinase-negative, albino mice and white mice, the latter completely lacking follicular melanocytes. The liver, kidney and brain of mice and the adrenals of rats also contained small amounts of these catechols. Both black and albino mice excreted free dopa and 5-S-cysteinyldopa, the amounts of which did not differ significantly in the two animals. It is suggested that oxidation mechanism(s) other than tyrosinase may participate in the synthesis of these catechols in proteins. Turnover of 5-S-cysteinyldopa-containing proteins may lead to the release of this catechol into blood and eventually to excretion into urine.