Reversible blocking of peptide amino groups by maleic anhydride.

Enzyme assays were carried out on foetuses removed near term and on rats from 0-100 days old, the animals beingweaned at 21 days. In agreement with Villee (1958) it was found that lipogenesis, as measured in vitro by incorporation of [14C]acetate into lipid of liver slices, was similar in foetal and adult tissue. Immediately after birth lipogenic activity fell and was maintained at a low level until 20-25 days. Thereafter it rose rapidly to attain a peak at 30-35 days and then declined steadily reaching adult levels at about 70 days. ATP: citrate lyase (EC 4.1.3.8) levels followed qualitatively those described for lipogenesis, exhibiting a substantial foetal level, a fall after birth, a rapid rise after 20 days, a maximum at 30-40 days and a fall to adult level at about 80 days. Malic enzyme (EC 1.1.1.40) behaves similarly except that no activity could be detected in the foetus, nor were appreciable quantities of enzyme found until the rise after 20 days. Glucose 6-phosphate dehydrogenase (EC 1.1.1.49) also gave a biphasic response similar to that of lipogenesis but the fall after birth was not as pronounced nor as rapid as in that case. Pyruvate kinase (EC 2.7.1.40) activity in the foetal liver is about half that in the adult. It is maintained at this low level until about 20 days after which it rises to reach adult level at 40-50 days. In confirmation of the results of Burch (1965) Fructose 1,6-diphosphatase (EC 3.1.3.11) activity was found to be very low in foetal liver and to rise very rapidly after birth to attain a maximum at 10 days, decreasing thereafter to reach adult levels at about 30 days. The results described are for male rats. Differences between male and female rats became apparent after 35 days for lipogenesis, malic enzyme and glucose 6-phosphate dehydrogenase; in each case females gave higher values. It is possible that these differences and the depression of enzyme levels found after 35 days are due to hormonal changes consequent to sexual maturation. The fluctuations in enzyme levels noted here, particularly striking at birth and weaning, are in marked contrast to the steady increases noted in the respiratory enzymes during development (Lang, 1965) and presumably reflect the control exerted on these enzymes by dietary and hormonal factors over this period.