PROTHROMBIN SYNTHESIS IN THE ADULT AND FETAL LIVER

Plasma levels of blood coagulation zymogens are lower in the newborn t han in the adult, with the lowest levels being in preterm infants. It is not known if the lower coagulation factor levels reflect difference s in synthesis, secretion or catabolism. Using a rabbit model we have compared prothrombin synthesis in the fetus and adult. In previous stu dies we attempted to compare transcription in the adult and fetal live r by extraction of mRNA, immobilization on a membrane and hybridizatio n with a labeled cDNA for rabbit prothrombin. Comparison was impaired by the markedly dissimilar composition of fetal and adult rabbit liver ; fetal liver is approximately fifty percent hematopoietic tissue even at term (1). In the present study, to obtain a more meaningful compar ison we have employed in situ hybridization to compare directly prothr ombin expression in adult and fetal liver. We report here that fetal l iver contains more prothrombin mRNA than does adult liver. We have fur ther compared prothrombin levels in protein extracts of adult and feta l liver and found that per microgram of extract, fetal liver contains as much prothrombin as does the adult. We conclude that the lower plas ma prothrombin levels in the fetus do not reflect a lower rate of synt hesis.