Skin color, aging, and plasma L-dopa levels

Although plasma levels of L-dopa are derived substantially from catecholamine-synthesizing tissues, melanocytes--which produce L-dopa as part of the melanin synthetic cascade--also may be a source of circulating L-dopa. We compared plasma L-dopa levels in albino subjects and in Caucasian and Black normal volunteers and patients with essential hypertension. DOPA levels were similar among the subject groups. Among Caucasian normal volunteers, L-dopa levels were negatively correlated with subject age (r = -0.30, P less than 0.05), whereas norepinephrine levels tended to increase with subject age (r = 0.25, 0.05 less than P less than 0.10), so that the L-dopa:norepinephrine ratio was highly negatively correlated with subject age (r = -0.50, P less than 0.01). Skin pigmentation does not contribute importantly to plasma L-dopa levels in humans. In contrast with levels of norepinephrine, L-dopa levels appear to decrease during normal aging.