Ultrastructural and biochemical studies of lipolysis by lipolysosomes in chick hepatocytes.

Lipolysosomes in chick hepatocytes were studied by biochemical and electron-microscopic techniques. Their lipolysis was investigated in vitro using the floating lipid fraction isolated from chick liver homogenates. Many lipid droplets were noted to be enveloped by a unit membrane with a partially thickened electron-dense layer containing some myelin-like structures, electron-dense material or electron-lucent droplets, close to the inner leaflet of the membrane. Cytochemically, the dense layer showed acid phosphatase activity, and thus the lipid droplets were identified as lipolysosomes. The lipolysosomes isolated in a floating lipid fraction retained the characteristic dense layer and the surrounding membrane, and the fraction showed little contamination by other organelles. Free fatty acids were released in the fraction after incubation at 37 degrees C, but not at 0 degree C. The reaction was completely inhibited by HgCl2, p-chloromercuribenzoate and esterastin, a potent inhibitor of acid lipase. These results suggest an involvement of acid lipase in lipolysis of the lipolysosomes.