I’rinfrd in USA. ELUCII)ATIOX OF THE RATE-LIMITING STEP IN NOREPINEPHRINE BIOSYNTHESIS IN THE IERFUSED GUINEA-PIG HEART

Recently it was shown that the isolated per-fused guinea-pig heart can synthesize norepi-nephrine from tyrosine (Spector et al., 1963), indicating that the heart contains all the en-zvmes necessa rv to form the neiirotransmitter from its dietary precursor. The rate of synthesis reported at that time was low (0.03 to 0.05 p.g/ g/hr) as only carrier-free tvrosine had been per-fused. In the present studies I)erftlsions were carried out with varying concentrations of the individual precursors of norepinephrine, i.e., tyrosine, (lopa and dopamine. Increasing con-centrations of all three precursors were found to yield increasing rates of norepinephrine synthesis. Only with tvrosine were maxima