Similar to prolyl-hydroxylases, lysyl-hydroxylases catalyse the hydroxylation of lysine, which is critical for collagen stability. The specifics of lysine hydroxylation are beyond the scope of this review, and are discussed elsewhere.80 Adding hydroxylation as a variable modification, improves identification and quantification of collagen levels in disease.3,68 Ascorbic acid (vitamin C) is a key cofactor for prolyl-4-hydroxylase, and its deficiency causes defects in collagen assembly.81 Inhibition of this enzyme has been shown to affect left ventricular remodelling after myocardial infarction in rats.82 In this study only proline:hydroxyproline ratios were assessed. MS provides assessment of hydroxylation with concomitant assignment to specific collagen types.