Development and characterization of an animal model of carnitine deficiency

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-hyd razine-3-propionate (THP), a butyrobetaine analogue, on butyrobetaine hydro xylase kinetics, and carnitine biosynthesis and body homeostasis in rats fe d a casein-based or a vegetarian diet. The K-m of butyrobetaine hydroxylase purified from rat liver was 41 +/- 9 mu mol.L-1 for butyrobetaine and 37 /- 5 mu mol.L-1 for THP, and THP was a competitive inhibitor of butyrobetai ne hydroxylase (K-i 16 +/- 2 mu mol.L-1). In rats fed a vegetarian diet, re nal 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 mu mol.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 mu mol.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 mu mol.g(-1)). Carnitine biosynthesis was blocked in THP-treated rats (-0.22 +/- 0.13 versus 0.57 +/- 0.21 mu mol.100 g(-1).day(-1)). Similar res ults were obtained in rats treated with the casein-based diet. THP inhibite d carnitine transport by rat renal brush-border membrane vesicles competiti vely (K-i 41 +/- 3 mu mol.L-1). Palmitate metabolism in vivo was impaired i n THP-treated rats and the livers showed mixed steatosis. Steady-state mRNA levels of the carnitine transporter rat OCTN2 were increased in THP-treate d rats in skeletal muscle and small intestine. In conclusion, THP inhibits butyrobetaine hydroxylase competitively, blocks carnitine biosynthesis in v ivo and interacts competitively with renal carnitine reabsorption. THP-trea ted rats develop systemic carnitine deficiency over three weeks and can the refore serve as an animal model for human carnitine deficiency.