Submitted on February 21, 2003
Revised on April 16, 2003
Accepted on April 15, 2003
Human apolipoprotein C-IV: isolation, characterization, and immunochemical quantification in plasma and plasma lipoproteins
Leila Kotite, Lin-hua Zhang, Zhonghua Yu, Alma L. Burlingame, and Richard J. Havel
Cardiovascular Researach Institute, University of California, San Francisco, San Francisco, CA 94143-0130
Corresponding Author: havelr{at}itsa.ucsf.edu
Apolipoprotein (apo) C-IV, the newest member of the low molecular weight apo C group , has been characterized in blood plasma of rabbits, in which it is a major apo C component, rich in proline (Zhang et al. (1996) J. Biol. Chem. 271: 1776-1783). Although the decoded sequence of mouse and human apo C-IV is known, apo C-IV has not been identified in blood plasma from these or other species. Rabbit apo C-IV exists in several sialoforms and the asialoform has an acidic isoelectric point. We show that apo C-IV is a basic protein in human, monkey and mouse plasma, present as a minor apo C component of VLDL. Human apo C-IV, isolated from apo VLDL by DEAE-cellulose chromatography and 2-dimensional electrophoresis, was identified by microsequencing 4 tryptic peptides. The protein exhibits two major isoforms; one is N-glycosylated, and both are variably sialylated. In normolipidemic plasma, greater than 80% of the protein is in VLDL (0.7% of total apo VLDL), with most of the remainder in HDL. The concentration of apo C-IV in plasma and lipoproteins of r< 1.21 g/ml is closely related to plasma triglyceride concentration up to 1770 mg/dl, varying from 0.1-1.9 mg/dl. Neither the human nor rabbit apo C-IV gene contains a typical TATA box in the 5’-flanking region, but the 5’-untranslated region of the rabbit gene contains a unique purine-rich sequence, GGGACAG(G/A), repeated 9 times in tandem, with an additional two within the 5’-flanking sequence. This sequence, functioning as a GAGA box that has been implicated in the transcription of a number of genes, may explain the higher level of expression of apo C-IV in rabbits.
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