Atleast9distinctpolymorphicformsofapolipoproteinsareknown.Theapolipoproteinsactasstabilizersoftheintactlipoproteinparticles.QuantitativemeasurementsofHDL,LDLandVLDLparticlesinhumanserumareoftenusedtoestimateanindividualsrelativeriskofcoronaryheartdisease.Inaddition,quantitativeimmunologicalmeasurementsofcertainapolipoproteins(especiallyA-1andB)havebeensuggestedtobemoreaccurateestimatorsofcoronaryheartdiseasethanmeasurementsoflipoproteinparticles(especiallyHDLandLDL).
Clusterin/ApolipoproteinJ(ApoJ)isthemajorsecretedproductofSertolicellsandisthoughttoplayacriticalroleinspermatogenesis.Theproteinwasshowntobeanormalconstituentofhumanbloodandisalsothoughttobeacontrolmechanismofthecomplementcascade.ItpreventsthebindingofaC5b-C7complextothemembraneofthetargetcellandinthiswayinhibitscomplement-mediatedcytolysis.
ApoJisoverexpressedinhumanprostateandbreastcancersandinsquamouscellcarcinomas.SuppressionofApolipoproteinJrendersthesecellssensitivetochemotherapeuticdrug-mediatedapoptosis.IthasbeenproposedthatelevatedlevelsofApolipoproteinJmaypromoteoncogenictransformationandtumorprogressionbyinterferingwithBAXproapoptoticactivities.