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VLDL Apolipoprotein B-100, a Potential Indicator of the Isotopic Labeling of the Hepatic Protein Synthetic Precursor Pool in Humans: Studies with Multiple Stable Isotopically Labeled Amino Acids^1,2,

USDA/Agricultural Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030

Four adult men received a 48-h constant intravenous infusion of [²H₄]lysine, [²H₃]leucine, L-[ring-¹³C₆]phenylalanine, and L-[1,2,3,-¹³C₃]alanine. Subjects ingested hourly meals for two 12-h periods, separated by two 12-h fasting periods. The isotopic enrichments of free amino acids in venous plasma and in VLDL apolipoprotein B-100 (apoB)-bound amino acids, plasma -keto isocaproic acid (-KIC) and plasma pyruvic acid (PYR) were measured by negative chemical ionization gas chromatography-mass spectrometry. By 7 h of infusion, all four amino acids achieved an equilibrium isotopic enrichment (EIE) in plasma and in apoB. In the fed state, the EIE of the amino acids in apoB was lower than that in plasma free amino acids. The ratio EIE-apoB:EIE-plasma differed significantly among amino acids in the fed state (alanine 0.30; lysine 0.64; leucine 0.70; phenylalanine 0.81). In the postabsorptive state, the EIE-apoB:EIE-plasma ratio rose significantly compared with the fed state (alanine 0.38; lysine 0.73; leucine 0.94; phenylalanine 1.05). Plasma PYR and apoB-alanine were in isotopic equilibrium irrespective of nutritional state. The EIE-apoB-leucine:EIE-plasma--KIC ratio rose from 0.75 in the fed state to near 1 in the postabsorptive state. We conclude that the contribution of systemic amino acids to apoB-100 synthesis is sensitive to nutritional state, and that systemic essential amino acids seem to be preferentially incorporated into apoB.

KEY WORDS: • liver • lipoprotein • protein synthesis • humans

¹ Supported in part by the Clinical Research Center of Texas Children's Hospital under grant number MO1-RR-00188. This work is a publication of the USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children's Hospital, Houston, TX, and has been funded with federal funds from the U.S. Department of Agriculture, Agricultural Research Service under Cooperative Agreement number 58-6250-1-003. The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products or organizations imply endorsement by the U.S. government.

² Preliminary accounts of this work have appeared: Patterson, B. W., Hachey, D. L., Motil, K. J., Booth, L. L., Cook, G. L., Reeds, P. J. & Klein, P. D. (1990) Enrichment of hepatic protein precursor pool varies with feeding status. Am. J. Clin. Nutr. 51: 521 (abs.), and Hachey, D. L., Patterson, B. W., Motil, K. J., Booth, L. B., Cook, G. L., Reeds, P. J. & Klein, P. D. (1990) Contribution of plasma amino acids to the hepatic protein synthetic pool varies with the amino acid. Am. J. Clin. Nutr. 51: 523 (abs.).

Manuscript received 21 May 1991. Revision accepted 24 September 1991.

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