Tangier disease. In vitro conversion of proapo-A-ITangier to mature APO-A-ITangier

Tangier disease is a disorder characterized by low levels of apo-A-I and high density lipoproteins. The defect in Tangier disease is an abnormal A-I apolipo protein, designated apo-A- ITangier . In normal subjects, apo-A-I is secreted as proapo -A-I with subsequent extracellular conversion to mature apo-A-I. The major form in normal plasma is mature apo-A-I with small amounts of proapo -A-I. In Tangier disease, proapo -A- ITangier is present in roughly equivalent concentrations compared to mature apo-A- ITangier . It has been proposed that the defect in Tangier disease is in the conversion of pro- to mature apo-A- ITangier . To test this, proapo -A-I was isolated from normal and Tangier subjects, and the conversion to the mature form by plasma from normal and Tangier subjects was analyzed. Incubation of radiolabeled normal proapo -A-I in normal plasma anticoagulated with heparin was associated with progressive conversion to mature apo-A-I over 24 h (initially 85% of the radioactivity was in the proapo -A-I isoform; at 24 h 33% radioactivity remained in the pro-isoform). Proapo -A- ITangier was also converted to the mature isoform during 24 h of incubation in normal plasma. Initially, 84% of radioactivity was in proapo -A- ITangier , and by 24 h the radioactivity in this isoprotein had decreased to 36%. A similar pattern of conversion was also observed when proapo -A- ITangier was incubated in Tangier plasma. The proteolytic conversion of both normal proapo -A-I and proapo -A- ITangier was unaffected by the serine protease inhibitors phenylmethylsulfonyl fluoride (1 mM) or aprotinin (200 Kallikrein-inactivating units/ml), but was inhibited by EDTA (0.1%). These results indicate that proapo -A- ITangier can be converted to mature apo-A- ITangier by the converting enzyme in normal plasma. In addition, plasma from a Tangier subject can convert both normal and Tangier proapo -A-I to the mature form. These results establish that proapo -A- ITangier can be rapidly converted to mature apo-A- ITangier , and there is no deficiency of the converting enzyme activity in Tangier disease.