Minor apolipoprotein mainly associated with HDL and to a lesser extent with VLDL. May also be associated with chylomicrons. Important determinant of plasma triglyceride (TG) levels by both being a potent stimulator of apo-CII lipoprotein lipase (LPL) TG hydrolysis and an inhibitor of the hepatic VLDL-TG production rate (without affecting the VLDL-apoB production rate) (By similarity). Activates poorly lecithin:cholesterol acyltransferase (LCAT) and does not enhance efflux of cholesterol from macrophages. Binds heparin (PubMed:17326667). {ECO:0000250|UniProtKB:Q8C7G5, ECO:0000269|PubMed:11588264, ECO:0000269|PubMed:12899628, ECO:0000269|PubMed:15528295, ECO:0000269|PubMed:17326667}. This is the function of ENSG00000110243 (APOA5, apolipoprotein A5).