Yip M C, Knox W E
Biochem J. 1972 May;127(5):893-9. doi: 10.1042/bj1270893.
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.
由于乳腺中存在精氨酸酶，哺乳期乳腺有大量鸟氨酸生成的潜力。在哺乳期后期，乳腺中精氨酸酶的活性高于除肝脏外的任何大鼠组织。由于该组织中不存在尿素循环中鸟氨酸进一步反应所需的两种酶，即氨甲酰磷酸合成酶I和鸟氨酸氨甲酰转移酶，因此可以排除尿素循环的存在。相反，氨甲酰磷酸合成酶II在哺乳期早期出现，与天冬氨酸氨甲酰转移酶和DNA的积累相关，这与这些酶在嘧啶合成中的作用一致。这些事实表明，除了其在尿素循环中的已知功能外，精氨酸酶还有其他生理作用。乳腺中鸟氨酸转氨酶的显著活性与精氨酸酶密切平行发展。通过这种反应，鸟氨酸可以转化为谷氨酸半醛，随后转化为脯氨酸。因此，哺乳期乳腺的酶组成适合于完整乳腺中已知发生的将精氨酸主要转化为脯氨酸的过程。
