**5.2 Apolipoprotein and lipid metabolism**

Possible correlations between apolipoprotein levels (Apo C-III, Apo C-I and Apo C-II) with dyslipidemia and cardiovascular disease were presented in [86]. Apolipoproteins function as the structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Apolipoproteins exhibit proteoforms associated with nucleotide polymorphisms (SNPs) and posttranslational modifications such as glycosylation, oxidation and sequence trunked [86]. The human apo Cs are protein constituents of chylomicrons, VLDL and

HDL. The protein APO C-III has 79 amino acids and can be glycosylated in the residue of Threonine 48. Initially, four APO C-III isoforms were identified by mass spectrometry and later 12 proteoforms. These proteoforms differ by absence of glycosylation (APO C-III Oa), glycosylation (APO C-III Ob), addition of one or two sialic acid residues (APO C-III 1, APO C-III 2) or addition of fucose at glycosylation sites. There are also truncated proteoforms due to amino acid substitution. Increases in APO C-III2 levels are associated with a reduction in TG and LDL levels, and perhaps this is a possible mechanism for dyslipidemia processes and reduced risk of cardiovascular disease (CVD) [86].
