γ-butyrobetaine in tissues and serum of fed and starved rats determined by an enzymic radioisotopic procedure

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Abstract
A method for the determination of picomole quantities of .gamma.-butyrobetaine and its application for the determination of .gamma.-butyrobetaine distribution in tissues are described. The method is based on the quantitative conversion of .gamma.-butyrobetaine into carnitine by using a 50-60%-saturated-(NH4)2SO4 fraction of rat liver supernatant as the source of .gamma.-butyrobetaine hydroxylase [4-trimethylaminobutyrate, 2-oxoglutarate:oxygen oxidoreductase (3-hydroxylating), EC 1.14.11.1]; the carnitine formed is then measured enzymically. The mean .gamma.-butyrobetaine content, as nmol/g wet wt of tissue, ranged from a low of 4.6 in livers to a high of 12.3 in hearts of normal fed male adults rats. Starvation for 48h did not affect the .gamma.-butyrobetaine concentration in serum, liver and brain, but that in skeletal muscles, kidney and heart was increased. These data are in line with the present views that most tissues are able to produce .gamma.-butyrobetaine, and show that starvation enhances the synthesis and/or the retention of the compound in many tissues. The observed high affinity of .gamma.-butyrobetaine hydroxylase for .gamma.-butyrobetaine (Km 7 .mu.M), the high activity of this enzyme and the low concentration of .gamma.-butyrobetaine in liver indicate that .gamma.-butyrobetaine availablity is one of the factors that normally limit carnitine synthesis.