BIOGENESIS OF ENDOPLASMIC RETICULUM MEMBRANES II. Synthesis of Constitutive Microsomal Enzymes in Developing Rat Hepatocyte

Thc constitutivc enzymes of microsomal mcmbrancs wcrc invcstigatcd during a pcriod of rapid ER dcvclopmcnt (from 3 days bcforc to 8 days aftcr birth) in rat hcpatocytcs. Thc activitics studicd (clcctron transport cnzymcs and phosphatascs) appcar at different times and incrcasc at diffcrcnt ratcs. Thc incrcasc in the cnzymc activitics tcstcd was inhibited by Actinomycin D and puromycin. G-6-Pasc and NADPH-cytochromc c rcductasc activities appcarcd first in thc rough microsomcs, and subscqucntly in smooth microsomcs, cvcntually reaching a uniform conccntration as in adult livcr. The cvidcncc suggcsts that thc cnzymcs arc synthcsizcd in the rough part, then transfcrrcd to thc smooth part, of the ER. Changcs in thc fat supplcmcnt of the maternal diet brought about changcs in thc fatty acid composition of microsomal phospholipids but did not influcncc thc enzymic pattcrn of thc suckling. Microsomcs from 3-day-old and adult rats losc 95 % of PLP and 80 % of NADHcytochromc c rcductasc activity after acctonc-H20 (I0: l) extraction. Howcvcr, onc-half thc original activity could bc rcgaincd by adding back phospholipid miccllcs prcparcd from purificd phospholipid, or from lipid extracts of heart mitochondria, or of liver microsomcs of 8~ay or adult rats, thus demonstrating an activation of thc enzymc by nonspccific phospholipid. Thc rcsults suggest that during dcvclopmcnt the cnzymic pattcrn is not influcnccd by the fatty acid or phospholipid composition of ER mcmbranes. I N T R O D U C T I O N In the preceding paper (1), we have described the rapid development of the ER 1 that occurs in rat 1 Abbreviations used are: NAD and NADH, oxidized and reduced nicotinamide adenine dinucleotide; NADP and NADPH, oxidized and reduced nicotinamide adenine dinucleotide phosphate; ER, endoplasmic reticulum; DOC, deoxycholate; NT, neotetrazolium chloride; PCMB, p-chloromercuribenzoate; IDP, inosine diphosphate; ATP, adenosine triphosphate; G-6-Pase, glucose-6-phosphatase; PLP, phospholipid; FA, fatty acid; Tris, Tris(hydroxymethyl) aminomethane. hepatocytes immediately before and after birth. In the fetus, the newly formed membranes are primarily of the rough type, while in the newborn the production of smooth membranes predominates. During this period, there are no changes in the phospholipid composition of microsomal membranes, but large changes in the fatty acid composition of phospholipids do occur. The aim of this part of the investigation was to look into the production of membrane proteins, primarily constitutive microsomal enzymes, during the preand postnatal periods. The results 97 on A ril 9, 2017 D ow nladed fom Published July 1, 1966