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

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%-satd.-(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 adult rats. Starvation for 48 h 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 this compound in many tissues. The observed high affinity of gamma-butyrobetaine hydroxylase for gamma-butyrobetaine (Km 7 microM), the high activity of this enzyme and the low concentration of gamma-butyrobetaine in liver indicate that gamma-butyrobetaine availability is one of the factors that normally limit carnitine synthesis.