In vivo regulation of phenylalanine hydroxylation to tyrosine, studied using enrichment in apoB-100
Roberts, S. A.
Ball, R. O.
Pencharz, P. B.
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Rafii, M., McKenzie, J., Roberts, S., Steiner, G., Ball, R. & Pencharz, P. (2008) In vivo regulation of phenylalanine hydroxylation to tyrosine, studied using enrichment in apoB-100, AJP: Endocrinology and Metabolism, vol. 294, , pp. E475-E479,
Phenylalanine hydroxylation is necessary for the conversion of phenylalanine to tyrosine and disposal of excess phenylalanine. Studies of in vivo regulation of phenylalanine hydroxylation suffer from the lack of a method to determine intrahepatocyte enrichment of phenylalanine and tyrosine. apoB-100, a hepatic export protein, is synthesized from intrahepatocyte amino acids. We designed an in vivo multi-isotope study, [15N]phenylalanine and [2H2]tyrosine to determine rates of phenylalanine hydroxylation from plasma enrichments in free amino acids and apoB-100. For independent verification of apoB-100 as a reflection of enrichment in the intrahepatocyte pool, [1-13C]lysine was used as an indicator amino acid (IAA) to measure in vivo changes in protein synthesis in response to tyrosine supplementation. Adult men (n = 6) were fed an amino acid-based diet with low phenylalanine (9 mgkg-1day-1, 4.54 _molkg-1,h-1) and seven graded intakes of tyrosine from 2.5 (deficient) to 12.5 (excess) mgkg -1day-1. Gas chromatography-quadrupole mass spectrometry did not detect any tracer in apoB-100 tyrosine. A new and more sensitive method to measure label enrichment in proteins using isotope ratio mass spectrometry demonstrated that phenylalanine hydroxylation measured in apoB-100 decreased linearly in response to increasing tyrosine intake and reached a break point at 6.8 mgkg-1day-1. IAA oxidation decreased with increased tyrosine intake and reached a break point at 6.0 mgkg-1day-1. We conclude: apoB-100 is an accurate and useful measure of changes in phenylalanine hydroxylation; the synthesis of tyrosine via phenylalanine hydroxylation is regulated to meet the needs for protein synthesis; and that plasma phenylalanine does not reflect changes in protein synthesis. Copyright 2008 the American Physiological Society.