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This organelle is particularly prominent in cells of the inner zones of the adrenal and fluctuates in amount and configuration in response to hormonal stimulation and sterol levels ( 4– 7). Many of the enzymes for sterol and steroid synthesis are localized in the smooth-surfaced endoplasmic reticulum ( 2, 3).
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The ratio of synthesis to uptake is dependent on the species, cell type, and functional state (see Ref. These cells synthesize cholesterol as a precursor for steroid hormones or take up this substrate from plasma lipoproteins.
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STEROID-SECRETING CELLS are characterized by abundant smooth endoplasmic reticulum (SER). We hypothesize that the broadly distributed translocon and oligosaccharyltransferase proteins participate in local synthesis and/or quality control of membrane proteins involved in cholesterol and steroid metabolism in a sterol-dependent and hormonally regulated manner. Confocal microscopy revealed the proteins to be distributed throughout the abundant tubular endoplasmic reticulum in these cells, which is predominantly smooth surfaced. This shifts the paradigm for distinction between smooth and rough endoplasmic reticulum. We further demonstrate that these smooth microsomal subfractions are capable of effecting cotranslational translocation, signal peptide cleavage, and N-glycosylation of newly synthesized polypeptides. In this study, we demonstrate that adrenal smooth microsomal subfractions enriched in smooth endoplasmic reticulum membranes contain high levels of translocation apparatus and oligosaccharyltransferase complex proteins, previously thought confined to rough endoplasmic reticulum. Yet they have relatively little morphologically identifiable rough endoplasmic reticulum, presumably required for synthesis and maintenance of the smooth membranes. Steroid-secreting cells are characterized by abundant smooth endoplasmic reticulum whose membranes contain many enzymes involved in sterol and steroid synthesis.