The Development of Some Enzymes of Aminoacid Metabolism in the Human Liver

Abstract

Besides problems concerned with specific inborn errors of metabolism the enzymes of aminoacid metabolism have a twofold interest to the pediatrician. The fundamental question of mammalian enzyme development and regulation of enzyme ‘induction’ has been studied intensively during recent years with respect to some of the enzymes involved in amino-acid metabolism such as tyrosine-transaminase (I, 2, 3). Furthermore from a nutritional and clinical point of view the maturity of enzymes involved in aminoacid synthesis and metabolism is of importance during the perinatal and neonatal period, since it has become apparent that the fetus and the newborn infant are metabolically different than the mature older organism of the same species and consequently the young organism cannot synthesize and metabolize some aminoacids as effectively as later. This leads to problems of differential diagnosis during early postnatal life especially with respect to premature infants which may show high levels of e.g. tyrosine in their blood due to transient low activity in the liver of enzymes oxidizing tyrosine (4). Furthermore the immaturity of some aminoacid synthesizing enzymes must be kept in mind with respect to artificial nutrition of premature infants. It has been known for some time that histidine is essential during human infancy (5). Recently it has been shown that also cyst(e)ine may be essential in the human fetus and premature infant (6).