The potential for a considerable formation of ornithine exists in lactating mammary gland because of its arginase content. Late in lactation arginase reaches an activity in the gland higher than that present in any rat tissue except liver. Occurrence of the urea cycle can be excluded since two enzymes for the further reaction of ornithine in the cycle, carbamoyl phosphate synthetase I and ornithine carbamoyltransferase, are both absent from this tissue. Instead, carbamoyl phosphate synthetase II appears early in lactation, associated with accumulation of aspartate carbamoyltransferase and DNA, consistent with the proposed role of these enzymes in pyrimidine synthesis. The facts require another physiological role for arginase apart from its known function in the urea cycle. Significant activity of ornithine aminotransferase develops in mammary gland in close parallel with the arginase. By this reaction, ornithine can be converted into glutamic semialdehyde and subsequently into proline. The enzymic composition of the lactating mammary gland is therefore appropriate for the major conversion of arginine into proline that is known to occur in the intact gland.

An enzyme which catalyzes the transfer of galactose from UDP‐galactose to galNAc‐gal‐glc‐ceramide is described. The enzyme is found mainly in the nervous tissue of tadpole (Taylor and Kollros stage 17), adult frog, adult and 8 day old rat. The enzymic activity is localized in the 11,500 xg, 20,000 xg and 100,000 xg particles. The UDP‐galactose: galNAc‐gal‐glc‐ceramide from the particles by treatment with sodium desoxylcholate and Triton X‐100. The pH optimum for the solubilized enyme is between 6.8 and 7.0 in cacodylate buffer, and the Km is 4.25 ×10−5 M. The enzymic reaction is proportional to time for 4 hr and to the amount of protein added. The product of the transferase reaction, using galNAc‐gal‐glc‐ceramide. A pathway for the biosynthesis of brain gangliosides requiring UDP‐galactose: galNAc‐gal‐glc‐ceramide galactosyl transferase is proposed.

Ornithine aminotransferase was purified from rat kidney by sonication of the isolated kidney mitochondria followed by ammonium sulfate fractionation of the solubilized protein. The ammonium sulfate precipitate was washed with distilled water and the insoluble material was extracted with 0.01 M-phosphate buffer containing 0.016 mMpyridoxal phosphate. This treatment preferentially solubilized the ornithine aminotransferase. The final preparation was purified 550-fold with a specific activity of 23.7 u/mg protein and a yield of 56%. One protein band was demonstrated by polyacrylamide gel disc electrophoresis. The molecular weight of the purified enzyme was estimated to be 165,000 by gel filtration on Sephadex G-200 column. The purified kidney enzyme was specific for L-ornithine as an amino group donor and a-oxoglutarate as an amino group acceptor. The Km for L-ornithine and aoxoglutarate was 5.9 mmol/1 and 1.0 mmol/1 respectively. The pH optimum was between 7.6 and 7.8. Kidney ornithine aminotransferase was induced by estrogen treatment. The increase in activity was due to an increase in enzyme amount as indicated by the augmentation in the specific activity of the mitochondrial preparation and in the yield of the purified enzyme. Liver ornithine aminotransferase was not affected by estrogen. The enzyme from liver could not be distinguished from the kidney enzyme by chemical or physical methods. Therefore, the variable response to estrogen seems to be organ-specific.