Comprehensive profiling of the competitive interplay among carbamylation, glycation, and enzymatic cross-links in human bone.

Carbamylation-derived products (CDPs) are implicated in protein dysfunction but remain understudied in human bone. We profiled homocitrulline (HCit, the predominant CDP), advanced glycation end-products (AGEs), and enzymatic cross-links in human cancellous bone specimens (n = 211). Homocitrulline accumulated substantially, surpassing carboxymethyl-lysine. Multivariable regression identified male sex and blood urea nitrogen (BUN) as independent determinants of HCit. This suggests men face a "dual burden" of renal-dependent accumulation and physiological urea loading driven by greater muscle mass. We observed a shift in the modification balance where higher BUN increased the HCit fraction while decreasing the AGEs-HCit ratio. Functionally, HCit correlated negatively with immature cross-links. Crucially, partial coefficient of determination (R2) analysis revealed that carbamylation and glycation act as concurrent determinants of cross-link impairment, with HCit exerting a stronger inhibitory effect than AGEs specifically in patients with mild BUN elevation (≥21 mg/dL). These findings demonstrate that carbamylation is a pervasive modification that actively interferes with collagen maturation, posing a significant threat to bone quality even in non-dialysis populations. Monitoring the carbamylation-glycation balance may offer new insights into bone fragility, especially in aging men and early-stage CKD.