Benders'dictionary of nutrition and food technology

370Tyrosinase (monophenol oxidase, EC 1.14.18.1) is a copperdependentenzyme in mammalian tissues, responsible for formationof melanin.Polyphenol oxidase (catechol oxidase, laccase EC 1.10.3.2) isa calcium-dependent enzyme responsible for browning of cutfruit and vegetables. It acts on catechols and other polyphenols,including flavonoids. Inhibited by chelating compounds such asedta, also by sulphites and cysteine.phenolphthalein See laxatives.phentermine An anorectic (appetite suppressant) drug formerlyused in the treatment of obesity, especially in combination withfenfluramine (fen-phen); withdrawn in 1995 in response toreports of heart valve damage.phenylalanine An essential amino acid; abbr Phe (F), M r 165.2,pK a 2.16, 9.18, codons UUPy. In addition to its role in proteinsynthesis, it is the metabolic precursor of the non-essential aminoacid tyrosine (and hence noradrenaline, adrenaline and thethyroid hormones).Tyrosine in the diet spares phenylalanine, soreducing the requirement.o-phenylene diamine Reagent used for determination of totalvitamin c after oxidation to dehydroascorbic acid with iodine;forms a fluorescent quinoxaline derivative with dehydroascorbicacid.phenylethylamine The amine formed by decarboxylation of theamino acid phenylalanine.phenylisothiocyanate See edman reagent.phenylketonuria A genetic disease affecting the metabolism ofphenylalanine. Phenylalanine is normally metabolised to tyrosine,catalysed by phenylalanine hydroxylase (EC 1.14.16.1).Impairment of this reaction leads to a considerable accumulationof phenylalanine in plasma and tissues (up to 100 timesthe normal concentration) and metabolism to phenylpyruvate,phenyllactate and phenylacetate, collectively known asphenylketones, which are excreted in the urine. The very highplasma concentration of phenylalanine causes disruption ofbrain development, and if untreated there is severe mentalretardation.Infants are screened for phenylketonuria shortly after birth(by measurement of plasma phenylalanine); treatment is by verystrict limitation of phenylalanine intake, only providing sufficientto meet requirements for protein synthesis. Once brain developmentis complete (between the ages of 8 and 12 years), dietaryrestriction can be relaxed to a considerable extent, since highconcentrations of phenylalanine seem to have little adverseeffect on the developed brain. There may, however, be benefits