UDP-Glucuronyltransferase (GT) activity increases in chick embryo liver during culture from zero to a steady-state level at or above adult values. The GT activity (o-aminophenol as acceptor) is located entirely in the membranes of the endoplasmic reticulum (ER) and the question arises whether ER increases along with GT. Earlier work showed that the synthesis and degradation rates of GT can be varied in culture over wide ranges by choosing embryo livers of different ages and both phenobarbital. In the present study we measured the GT activities and the concentrations of ER (using stereologic methods) in 5- and 11-day embryo liver during culture with and without phenobarbital. We found that GT and ER always increased in a constant ratio of 2.2 X 10(-9) U of GR activity per square micrometer of membrane, suggesting that the synthesis and degradation of GT are coupled to the synthesis and degradation of ER. A general structure for ER is proposed to explain this finding.

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