Phenylalanine

L-Phenylalanine (be ta-pheny I alanine, alpha-aminohydrocinnamic acid, alpha-amino-bcta-pheny I prop ionic acid; one-letter code F: molecular weight 165) is an essential amino acid with a nitrogen content of 8,5%.

Figure N.41 L-Phenyl a la mm:

Abbreviations

BHj 5,6,7,8-tetrahydrobioptenn

LAT2 L-type amino acid transporter 2

Phe L-phenylalanine

PKU phenylketonuria

PLP pyndoxal 5-phosphate

TATl T-type amino acid transporter 1

Tyr L-tyrosine

Nutritional summary

Function; The essential amino aetd L-phenylalanine (Pile) ts needed tor the synthesis of proteins, catecholamines, and melanin: it is also an important precursor of the amino acid L-tyrosine (Tyr). Phe is used as an energy fuel: its complete oxidation requires biopterin, ascorbate. thiamin, riboflavin, niacin, vitamin B6. pantothenate, lipoate, ubiquinone, magnesium, and iron.

Food sources: Adequate amounts are consumed when total protein intakes meet recommendations.

Requirements: The combined daily adult requirement for Phe and Tyr is 39mg'kg (Young and Borgonha, 2000).

Deficiency: Prolonged lack of Phe. as of all essential amino acids, or a lack of protein causes growth failure, toss of muscle mass, and organ damage. Excessive intake: Adults with phenylketonuria (PKU) should not exceed the Phe requirement, particularly not during pregnancy. There have been anecdotal reports of illness alike to eosinophils myalgia syndrome and of seizures following use of manufactured Phe.

Dietary sources

The protein in legumes, such as soybeans (5.5%), eggs (5.3%), and rice (5.3%). has a slightly higher content than cow milk (4.8%). wheat (4.9%), and corn (4.4%). Pork (4.0%). beef (3.9%). and chicken (3.9%) contain slightly smaller percentages. In the end howev er, total protein content is the main determinant of Phe intakes. A typical US diet provides 40 mg kg body weight or more (Gibson et al.. 2002).

A commonly used synthetic source of Phe is aspartame (N-l.-alpha-aspartyl-L-phenylalanine 1-methyl ester), which provides about 40 mg Phe per serving of sweetener.

Digestion and absorption

Chewing of foods in the mouth, denaturation by hydrochloric acid and various proteases including nensin in the sinmarh and hviImlvsk hv nanrnvitir and f*mr>ritf

Figure 8.32 Intestina I absorption or (.-phenylalanine

for absorption. Di- and tripeptides arc taken up via the hydrogen ion peptide c o transponer 1 (PepTl, SIC15A1) and. to a lesser extent, 2<PcpT2, S1.C15A2). Transport of free Phe is mediated by the sodium-amino acid cotransport system B° (Avissaref til.. 2001). Exchange for other neutral amino acids via the sodium-independent transporter complex BATl/b° -rBAT (SLC7A9-SLC3A1) extends the capacity oí the sodium-dependent transporters (Verrey et al.. 1999; Mizoguchi el al., 2001).

Nearly a third of the Phe taken up appears to be used by entcroeytes themselves, mostly for protein synthesis. 80% after conversion to tyrosine (Matthews et ai, 1993). ThcT-type amino acid transporter I I TAT I) facililates diffusion across the basolateral membrane of enterocytes in jejunum, ileum, and colon (Kim e/ ul., 2001). Additional transport capacity is provided by the 4F2-glycoprotein-anchored exchanger LAT2 (Rossier eta!., 1999: Rajan et ul.. 2000).

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