title: In vivo tyrosine hydroxylation rate in retina: effects of phenylalanine and tyrosine administration in rats pretreated with p-chlorophenylalanine.
abstract: p-Chlorophenylalanine was administered to rats to inhibit hepatic phenylalanine hydroxylase activity. Two days later, phenylalanine injection was noted to produce substantial increases in serum phenylalanine levels, and relatively modest increments in serum tyrosine levels. Rats injected with p-chlorophenylalanine 2 days earlier showed a normal light-induced activation of retinal tyrosine hydroxylase activity in vivo, measured as dihydroxyphenylalanine accumulation following pharmacologic inhibition in vivo of aromatic L-amino acid decarboxylase activity. In addition, tyrosine injection into p-chlorophenylalanine-treated rats in the light produced anticipated increments in retinal tyrosine hydroxylation rate, showing the enzyme to be functionally normal. The acute administration of phenylalanine (62.5-500 mg/kg i.p.) to p-chlorophenylalanine-treated rats produced dose-related increments in retinal phenylalanine. In vivo tyrosine hydroxylation rate in retina was normal at all doses below 300 mg/kg. However, at the highest dose (500 mg/kg), when retinal phenylalanine levels were almost 5-times normal tyrosine hydroxylation rate consistently fell (to about half-normal values). These results demonstrate that very large elevations in tissue phenylalanine levels do not stimulate tyrosine hydroxylation in vivo, and that at extremely high levels phenylalanine inhibits tyrosine hydroxylation rate.
authors.affiliation: Department of Psychiatry, University of Pittsburgh School of Medicine, PA 15213.
authors.forename: M H
authors.initials: MH
authors.lastname: Fernstrom
authors.forename: R L
authors.initials: RL
authors.lastname: Baker
authors.forename: J D
authors.initials: JD
authors.lastname: Fernstrom
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D010134:Fenclonine / Q000494:pharmacology*; D008297:Male; D010649:Phenylalanine / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012160:Retina / Q000187:drug effects / Q000201:enzymology*; D014443:Tyrosine / Q000378:metabolism*; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism*
chemical_list: D014443:Tyrosine; D010649:Phenylalanine; D014446:Tyrosine 3-Monooxygenase; D010134:Fenclonine