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The dynamics of cellular growth are of prime importance to the biochemist. The dynamic state of lipids can be studied by employing radioactive substrates or stable isotope-labeled substrates. This paper illustrates major factors which may effect the incorporation of precursor substances into body constitutents. These factors are: (a) age and species; (b) molar size of the body’s pool of precursor; (c) metabolic activity of the lipid being synthesized; (d) metabolic pathways; (e) route of administration of the precursor; (f) the nature of the precursor, i. e., molecular size, ionic or nonionic, water soluble vs. lipid solubility, and micelle formation; and (g) the influence of hormones and drugs. The synthesis and turnover of cerebrosides and gangliosides in the rat are used to illustrate these seven factors.

A mixture of UDP-(N-acetyl)glucosamine-114C and UDP-(N-acetyl)galactosamine-114C (7∶3) served as a substrate to demonstrate in young rat brain tissue the presence of enzymes which catalyzes the reactions: UDP-glcNAc⇌UDP-galNAc (E.C.5.1.3) and UDP-galNAc+ gal-glc-cer→ galNAc-gal-glc-cer+UDP. The glycolipid acceptor specificity was examined and preliminary kinetic data were obtained for the UDP-(N-acetyl)galactosamine: gal-glc-ceramide N-acetyl galactosamine transferase. The products of the reaction were identified. The relationship of this reaction to ganglioside biosynthesis is discussed.

Lipids were extracted from beef retina by chloroform-methanol (2∶1); the gangliosides were removed from the total lipid extract by partitioning into water and chromatographing on thin layer plates coated with silica gel. The analytical methods are described for estimating ganglioside components, i.e., N-acetyl neuraminic acid, hexoses, hexosamine, sphingosine and fatty acids, in the presence of silica gel. Major gangliosides present in beef retina have been tentatively identified as follows: a ganglioside containing two N-acetyl neuraminyl groups but no hexosamine; two gangliosides containing two N-acetyl neuraminyl groups and one hexosamine; and a ganglioside with three N-acetyl neuraminyl moieties and one hexosamine.

Tritium labeled galactosyl ceramide was administered intravenously to three male monkeys (Macaca mulatta). Blood, urine, feces and organs obtained at autopsy were examined for radioactivity after 1 hr, 8 hr and 37 days. The radioactive galactosyl ceramide was taken up by the reticuloendothelial system. Very little radioactivity appeared in brain tissue. The radioactivity was excreted primarily in urine.