Tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) enzyme and mRNA activity were not detectable in day 20 fetal rat liver. Precocious induction of catalytic activity by in utero injection of dibutyryl cAMP was a direct consequence ofthe de novo appearance oftranslatable tyrosine aminotransferase mRNA. In contrast, in utero injection ofhydrocortisone acetate failed to elicit fetal liver enzyme activity. This failure was due to the inability of the steroid hormone to induce the appearance of tyrosine aminotransferase mRNA activity. In fetal rat liver explants, either compound was capable of stimulating the synthesis of adult levels of enzyme and mRNA. However, catalytic and mRNA activity comparable with that seen in vivo 24 hr after birth required the concerted action of both inducers.One of the central problems in the study of development is the determination of the factors responsible for differential gene expression and the appearance of tissue-specific proteins. The enzymatic differentiation of developing rat liver has proved to be a useful model for these studies. During development, most rat liver enzymes accumulate rapidly and in discrete groups or clusters: the late fetal cluster (16-22 days ofgestation), the neonatal cluster, and the late suckling cluster (1).Tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) is one of the enzymes appearing in the neonatal cluster (1). Enzyme activity, virtually absent in fetal rat liver, starts to increase 2 hr after birth, reaching a maximum of at least twice the adult level by 12 hr. The catalytic activity then decreases to adult level by 2 days after birth (2, 3). Premature appearance of tyrosine aminotransferase can be elicited in fetal rats by in utero injection of glucagon (4, 5) or dibutyryl cAMP (6); glucocorticoid injection does not precociously induce catalytic activity (4, 6). However, all three compounds induce enzyme activity in vitro (7-9).As part of our studies on the regulation of tyrosine aminotransferase gene expression, we are investigating the process responsible for the appearance of tyrosine aminotransferase in newborn rats. We present evidence that the appearance of enzyme activity after precocious induction in utero is a direct consequence of the de novo accumulation of mRNA coding for tyrosine aminotransferase. In addition, we have used fetal rat liver explants to correlate tyrosine aminotransferase mRNA and enzyme activity during induction by dibutyryl cAMP and hydrocortisone acetate.MATERIALS AND METHODS Chemicals. Cortef acetate (hydrocortisone acetate, 50 mg/ ml) was provided by Upjohn. BGJb medium, Fitton-Jackson modification, and penicillin/streptomycin (penicillin, 10,000 units/ml; streptomycin, 10,000 Ag/ml) were from GIBCO.Translation grade [3S]methionine (specific activity, 1350-1450 Ci/mmol; 1 Ci = 3.7 X 101°becquerels) was from Amersham. Cordycepin, protein A-Sepharose, and other biochemicals were from Sigma.Animals. Sprague-Dawley (CD) rats from Charles River Br...

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The mechanisms for the initiation of sperm motility have been poorly understood until recently. Immobilin is a novel mucin glycoprotein of high molecular weight found in the cauda epididymis of the rat that, at concentrations equivalent to those found in native cauda epididymal fluid, reversibly inhibits sperm motility. In this study, immobilin was purified from rat cauda epididymal fluid to apparent homogeneity and used to generate polyclonal antibody in rabbits. The antibody was characterized by immunoblotting, and immunofluorescence was used to localize immobilin in paraffin sections of components of the reproductive system of adult male rats. Immobilin was not detectable in the efferent duct and was first detectable in the apical portion of some epithelial cells of the initial segment of the caput epididymis. Immobilin was detectable intracellularly only in cells of the caput epididymis. In the corpus and cauda epididymis immobilin was detectable only in the lumen of the tubules. Immunoprecipitation of immobilin radiolabeled in vitro confirmed that immobilin biosynthesis in the adult rat is restricted to the caput epididymis. Principal cells in the caput epididymis synthesize immobilin and secrete it into the lumen of the tubules to travel with the sperm into the cauda.

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