TYROSINE METABOLISM IN USERS OF ORAL-CONTRACEPTIVES

Brain noradrenaline takes part in the regulation of several brain func tions. The formation of brain noradrenaline depends on brain tyrosine (Tyr) levels, which associates with the ratio in plasma of Tyr to othe r large, neutral amino acids (LNAA): Tyr metabolism has been studied i n users of the new generation combined oral contraceptives (OC) and co mparable controls at the follicular, mid-cycle, and luteal phases of t he menstrual cycle. OC users showed significantly increased plasma Tyr transaminase activity, and significantly decreased plasma Tyr and Tyr /LNAA levels at mid-cycle and luteal phase, whereas plasma total 3-met hoxy-4-hydroxyphenylglycol (MHPG) was not affected. Following an oral protein load, the area under the curve in plasma of Tyr and Tyr/LNAA i n OC users at the luteal phase were 43 % and 29 %, respectively, of co ntrol levels. The results suggest that the decreased Tyr availability to the brain in OC users may result in a substrate-limited reduction o f brain noradrenaline formation, which, secondarily, may contribute to disturbances of mood, coping mechanisms, and appetite in susceptible subjects.