Cytochemical studies of lipid metabolism: immunogold probes for lipoprotein lipase and cholesterol.

In this article, cytochemical methods are presented for the study of lipid metabolism both in normal cells and in mutant cells with genetic disorders characterized by abnormal lipid metabolism. The benefit of using an immunocytochemical approach to the study of lipase in tissues is discussed, and a review is presented of the results on immunolocalization of lipoprotein lipase in cardiac tissue of normal mice. Immunocytochemical techniques are applied to the study of lysosomal proliferation in hepatocytes from liver of mutant mice with a genetic defect responsible for the lack of hepatic lipase and lipoprotein lipase activity in these animals. Localization of lipids in tissues with structural techniques has been an area of great interest to our laboratory for many years. Attention is called to the development of a technique for the visualization of fatty acids as a function of their ionization state and the production of fatty-acid myelin figures in membranes. Results on the use of filipin to detect unesterified cholesterol in membranes are reviewed. Filipin produces fluorescent filipin-cholesterol complexes but also perturbs cell membranes. Application of this cytochemical probe, in combination with immunocytochemistry of lysosomes, produced useful information on defects in low-density lipoprotein-derived cholesterol translocation in mutant human fibroblasts. Initial results on the application of immunological techniques to the study of cholesterol in lipid model systems indicate a novel approach, which may be applicable to specialized cell systems. Recent advances in cryoultramicrotomy and development of immunoprobes present valuable opportunities for the structural assessment of lipids and lipases in cell organelles and cell membranes.