Cellular and secreted lipoprotein lipase revisited.

Lipoprotein lipase (LPL) of adipose cells is present only in membrane compartments, mainly in the Golgi apparatus. LPL is a typical secretory protein which appears to be active as a homodimer. The process of LPL synthesis and maturation requires multiple steps. LPL is synthesized in the endoplasmic reticulum as an inactive monomer of Mr 51,000; a high-mannose, inactive monomer of Mr 55,500 is then formed. An active homodimer form, bearing two complex oligosaccharide chains per monomer of Mr 58,000, forms in the Golgi apparatus. This mature form, present in secretory vesicles, can be secreted constitutively or after exposure to heparin. A model is proposed in which LPL is present in secretory vesicles in a potentially active, condensed, or "polymerized" form. This model, which applies to various LPL-containing tissues in different species - including human - would explain the "activation" of LPL.