Similarities in properties, content, and relative rates of synthesis of fructose-P2 aldolase in livers of fed and starved rats

Developmental regulation of the hepatocyte receptor for galactose-terminated glycoproteins

The receptor which recognizes glycoproteins that have had their terminal sialic acids removed, thus exposing penultimate galactose residues (asialoglycoproteins), was examined for expression in rat liver during development. The level of asialoglycoprotein receptor binding activity in fetal rat livers was present in very low amounts but rose dramatically at the time of birth and reached adult levels by the second day after birth. Using immunoquantitation methods, it was found that the increased binding capacity of rat liver for asialoglycoproteins during development reflected accumulation of receptor molecules rather than activation of previously existing ones. The relative rates of synthesis of the predominant polypeptide of Mr 42,000 and the lesser abundant polypeptides of Mr 50,000 and 58,000 which comprise asialoglycoprotein receptor were found to increase in livers of fetuses near term and attain adult synthesis rates around birth. Thus, the accumulation of receptor protein molecules during development reflected increased synthesis of receptor polypeptides. These results suggest that the different gene products which code for the three forms of the receptor are coordinately expressed during development. Copurifying with asialoglycoprotein receptor during ligand affinity chromatography were polypeptides of Mr 25,000 and 27,000. These polypeptides display several characteristics similar to hepatic mannose binding lectin described by others. Onset of synthesis of the mannose binding lectin during development was analogous to asialoglycoprotein receptor but, in contrast, did not reach adult synthesis rates immediately after birth.

Characterization of transverse tubule membrane proteins: tentative identification of the Mg-ATPase

Vesiculated fragments of chicken skeletal muscle transverse tubule (TT) membranes were analyzed for their content of loosely associated and integral membrane proteins. Of particular interest was the identification of the magnesium-stimulated ATPase (Mg-ATPase), which is characteristically located in native isolated TT vesicles of chicken skeletal muscle [R. A. Sabbadini and V. R. Okamoto (1983) Arch. Biochem. Biophys. 223, 107-119]. A number of the proteins found in vesicular TT preparations were found to be extractable by a mild Triton-X100 treatment and were identified as aldolase, enolase, creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and pyruvate kinase. Approximately 60% of TT-associated protein was extracted with Triton, resulting in a twofold enrichment of the Mg-ATPase. Concommitantly, one core integral membrane protein possessing a Mr of 102,000 was enriched, suggesting that it is responsible for the Mg-ATPase activity present in chicken skeletal muscle TT membranes.