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-Butyrobetaine Hydroxylase Activity is Not Rate Limiting for Carnitine Biosynthesis in the Human Infant^1,2,

Department of Pediatrics and Program in Human Nutrition, University of Iowa College of Medicine, Iowa City, IA 52242

Carnitine biosynthesis was assessed in human infants by measuring changes in plasma carnitine concentration and rates of urinary carnitine excretion after infants were fed carnitine-free formulas with and without added -N-trimethyl-L-lysine or -butyrobetaine. This study was undertaken to test the hypothesis that carnitine biosynthesis in the human infant is regulated by substrate availability rather than activity of -butyrobetaine hydroxylase, the final enzyme in the carnitine biosynthetic pathway. Ten infants were fed carnitine-free formula supplemented with either 500 µM -N-trimethyl-L-lysine or 500 µM -butyrobetaine for 14 d. Plasma carnitine concentration and rate of urinary carnitine excretion were measured in infants before and after this period. Plasma carnitine concentration increased twofold when infants were fed -N-trimethyl-L-lysine and increased threefold when infants were fed -butyrobetaine. The rate of carnitine excretion doubled when infants were fed -N-trimethyl-L-lysine and increased 30-fold when infants were fed -butyrobetaine. Absorption of -N-trimethyl-L-lysine was verified by demonstrating increased urinary excretion of -N-trimethyl-L-lysine in infants fed this substrate. We conclude that -butyrobetaine hydroxylase activity is not rate limiting for carnitine biosynthesis in the human infant. Development of renal and hepatic -butyrobetaine hydroxylase activity was determined in necropsy tissue from individuals of various ages. It was verified that -butyrobetaine hydroxylase activity is developmentally regulated in the liver, but not in the kidney. The clinical relevance of this observation is diminished in view of the results of the in vivo studies of carnitine biosynthesis in infants.

KEY WORDS: • carnitine • -N-trimethyl-L-lysine • -butyrobetaine • metabolic regulation • human infant

¹ This study was supported by a Biomedical Research Support Grant (RR05372) from the National Institutes of Health. Ms. Olson was supported by a Training Grant (HD07287) from the National Institutes of Health.

² Presented in part at the Triennial Joint Meeting of the American Institute of Nutrition, American Society for Clinical Nutrition and Canadian Society for Nutritional Sciences, Davis, CA, July 20–24, 1986. Olson, A. L. & Rebouche, C. J. (1986) Carnitine biosynthesis from -N-trimethyllysine and -butyrobetaine in the human infant. J. Nutr. 116(6): xxx (abs. 58).

Manuscript received 20 November 1986. Revision accepted 2 February 1987.

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