Tyrosine hydroxylation is considered to be the rate-limiting step in 
catecholamine synthesis. It is also assumed that under usual conditions, 
tyrosine 3-monooxygenase (EC 1.14.16.2) (tyrosine hydroxylase - TH) is close to 
full saturation with its l-tyrosine substrate and hence that raising the 
availability of l-tyrosine does not substantially increase 
3,4-dihydroxyphenylalanine (DOPA) synthesis. We evaluated this in vivo by 
reverse dialysis of the aromatic-l-amino-acid decarboxylase (EC 4.1.1.28) 
inhibitor NSD-1015 (20μM) and selected concentrations of l- or d-tyrosine. In 
striatum, extracellular DOPA levels increased linearly (maximum 250% control) as 
l-tyrosine concentrations were raised from 0-1000μM. In medial prefrontal 
cortex, DOPA levels reached a maximum (300% control) at l-tyrosine 62.5-125μM 
but still remained significantly elevated (200% control) at higher l-tyrosine 
concentrations (250-500μM). At the l-tyrosine concentrations tested, DOPA levels 
were never below those of controls. d-tyrosine (62.5μM) did not affect DOPA 
levels. The degree to which the elevation of DOPA levels represents a net 
increase in tyrosine hydroxylation as opposed to heteroexchange of l-TYR for 
intracellular DOPA remains to be determined. However, one interpretation of the 
data is that under usual in vivo conditions brain TH may not be near full 
saturation with l-tyrosine and that mechanisms other than tyrosine hydroxylation 
may be more important in the acute regulation of brain catecholamine synthesis 
than previously appreciated. This would have implications for the 
pathophysiology and treatment of neuropsychiatric conditions in which 
dysregulation of DA transmission and l-tyrosine transport have been implicated.