Aspartate transcarbamylase activity was assayed in rat tissues, and the limitation caused in some tissues by carbamyl phosphate hydrolysis was avoided. Significant activity was found in the particulate cell fractions, especially in fetal and neoplastic tissues, as well as in soluble fractions. The enzyme of the particulate fraction was more sensitive to heat and had greater affinity for aspartate than had the soluble enzyme. Total aspartate transcarbamylase concentrations, relative to the liver standard, were generally higher in fetal tissues and decreased within 3 weeks after birth to normal adult levels. In the mammary gland, it rose and fell during the lactation cycle in parallel with cellular growth. In transplanted tumors, the relative enzyme concentration was significantly correlated with measured rates of growth in tumor volume over a 6-fold range of levels and rates. Relative concentrations per g in the faster tumors were similar to those in the fetal tissues that grew at similar rates.


This investigation was supported by USPHS Grants AM 00567 and AM-K6-2018 from the National Institute of Arthritis and Metabolic Diseases, by Grant HD-04532 from the National Institute of Child Health and Human Development, and by United States Atomic Energy Commission Contract AT(30-1)-3779 with the New England Deaconess Hospital.

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