Urinary galactosyl-hydroxylysine in postmenopausal osteoporotic women: A potential marker of bone fragility

Alterations of the collagen matrix, e.g., increased hydroxylation and glyco sylation of lysyl residues in collagen I, were found in human osteoporotic bone, and it was suggested that they could alter the mechanical properties of skeleton. To test this hypothesis, we evaluated the content of galactosy l-hydroxylysine (GHYL) in bone collagen, as assessed by its urinary excreti on, and related it to the occurrence of fracture. Two hundred and fifteen u nselected postmenopausal women with osteoporosis were divided in two subgro ups (comparable for age, age of menopause, bone mineral density, and bioche mical parameters of bone turnover) on the basis of the history of fragility fracture; 115 patients had suffered no fracture and 100 patients had suffe red one or more fractures 3 or more years before. Four urinary markers of b one turnover (hydroxyproline, cross-linked N-telopeptide, free deoxypyridol ine, and GHYL) were evaluated in all patients. There was: no difference bet ween the two groups with regard to all the parameters studied except for GH YL, which was significantly higher in the group with a history of fracture (1.35 +/- 0.82 mmol/mol of creatinine [Cr] versus 1.03 +/- <0.48 mmol/mol. Cr, p < 0.001); this marker did not correlate with other markers of bone re modeling in the fracture group, indicating a possible defect in bone collag en. In conclusion, provided that increased levels of urinary GHYL do reflec t overglycosylation of hydroxylysine in bone collagen, the GHYL may be cons idered a marker of bone collagen quality. Our results, showing higher urina ry GHYL in osteoporosis patients with fracture, seem to confirm this sugges tion.