Perinatal development of the tyrosine oxidizing system.

The enzymes of the tyrosine oxidase (TO) system of rat liver were measured during pre- and postnatal development. At all ages, the rate of tyrosine oxidation was limited by the activity of tyrosine aminotransferase (TAT). TAT of liver was undetectable from 4 days to 1 day before birth and was lower than p-hydroxyphenylpyruvate oxidase (pHPPO) and homogentisate oxidase (HO) postnatally. At birth, TAT, pHPPO, and HO were 2.9, 13, and 40% of adult values, respectively. Relative levels in fetal liver were HO greater than pHPPO greater than TAT. When enzyme activities were adjusted to compensate for dilution by hematopoietic cells, absolute activities increased, but other relationships remained the same. pHPPO rose at 15-17 days after birth and exceeded adult values at 31 days. HO was unchanged after birth. We conclude that each enzyme of TO follows a unique developmental path, with no concerted control of the entire TO system.