Development and characterization of an animal model of carnitine deficiency

Abstract

Mammals cover their carnitine needs by diet and biosynthesis. The last step of carnitine biosynthesis is the conversion of butyrobetaine to carnitine by butyrobetaine hydroxylase. We investigated the effect of N-trimethyl-hydrazine-3-propionate (THP), a butyrobetaine analogue, on butyrobetaine hydroxylase kinetics, and carnitine biosynthesis and body homeostasis in rats fed a casein-based or a vegetarian diet. The Km of butyrobetaine hydroxylase purified from rat liver was 41 ± 9 μmol·L-1 for butyrobetaine and 37 ± 5 μmol·L-1 for THP, and THP was a competitive inhibitor of butyrobetaine hydroxylase (Ki 16 ± 2 μmol·L-1). In rats fed a vegetarian diet, renal excretion of total carnitine was increased by THP (20 mg·100 g-1·day-1 for three weeks), averaging 96 ± 36 and 5.3 ± 1.2 μol·day-1 in THP-treated and control rats, respectively. After three weeks of treatment, the total carnitine plasma concentration (8.8 ± 2.1 versus 52.8 ± 11.4 μmol·L-1) and tissue levels were decreased in THP-treated rats (liver 0.19 ± 0.03 versus 0.59 ± 0.08 and muscle 0.24 ± 0.04 versus 1.07 ± 0.13 μmol·g-1). Carnitine biosynthesis was blocked in THP-treated rats (-20.22 ± 0.13 versus 0.57 ± 0.21 μmol·100 g-1·day-1). Similar results were obtained in rats treated with the casein-based diet. THP inhibited carnitine transport by rat renal brush-border membrane vesicles competitively (Ki 41 ± 3 μmol·L-1). Palmitate metabolism in vivo was impaired in THP-treated rats and the livers showed mixed steatosis. Steady-state mRNA levels of the carnitine transporter rat OCTN2 were increased in THP-treated rats in skeletal muscle and small intestine. In conclusion, THP inhibits butyrobetaine hydroxylase competitively, blocks carnitine biosynthesis in vivo and interacts competitively with renal carnitine reabsorption. THP-treated rats develop systemic carnitine deficiency over three weeks and can therefore serve as an animal model for human carnitine deficiency.