The total activity of adenine phosphoribosyltransferase/liver of mice remained constant from 1 to 16 days after birth despite a fourfold increase in liver weight. The total activity of this enzyme increased fivefold from 16 to 36 days and then remained relatively constant at least until 96 days after birth. Total hypoxanthine-phosphoribosyltransferase activity/liver steadily increased between 1 and 57 days after birth. 2. The mean K(m) of 5-phosphoribosyl pyrophosphate with adenine phosphoribosyltransferase was 10.1mum between 3 and 11 days, at 64 days and at 96 days after birth. Between 17 and 51 days the mean K(m) value was 3.0mum. The K(m) of 5-phosphoribosyl pyrophosphate with hypoxanthine phosphoribosyltransferase remained constant at 28.2mum between 2 and 64 days. 3. Adenine-phosphoribosyltransferase activity was stimulated between 15 and 83% by 60mum-ATP when extracts were made between 3 and 11 days, at 64 days or at 96 days after birth. Between 17 and 51 days ATP had little stimulatory effect on the activity of this enzyme. 4. AMP competed with 5-phosphoribosyl pyrophosphate in the reaction catalysed by adenine phosphoribosyltransferase. Liver extracts containing enzyme with a low value of K(m) for 5-phosphoribosyl pyrophosphate (3mum) had a K(m)/K(i) ratio approximately half that of extracts with a high value of K(m) (10mum). 5. The results indicate that two different forms of adenine phosphoribosyltransferase can exist in mouse liver at different stages of development. The physiological significance of these findings is discussed.
