Key enzymes of purine degradation and reutilization in human fetal liver and brain

The activities and kinetic parameters of 5'-nucleotidase, adenosine monophosphate (AMP) deaminase, and hypoxanthine/guanine phosphoribosyltransferase (HGPRT) were assayed in human fetal brain and liver. The apparent activity of 5'-nucleotidase decreases in the liver and increases in the brain with gestation. Its apparent Km is about 27 microM for AMP in the brain at term and liver throughout gestation but about 60 microM in the premature brain. The activity of (AMP) deaminase in the liver increases, while that in the brain does not significantly change with gestation. Its apparent Km for AMP is about 5 mM in the tissues studied. The activity of HGPRT increases with gestation in both the fetal brain and liver, and is altogether high in the fetal brain. Its apparent Km for hypoxanthine appears to be high, about 59 microM, in the tissues studied. The activities measured reflect the maximal capacities of the enzymes. Conclusions concerning their in vivo activities cannot, however, be based on studies employing disrupted cells.