Comprehensive Profiling of the Competitive Interplay Among Carbamylation, Glycation, and Enzymatic Cross-Links in Human Bone

Abstract 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). HCit 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 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 chronic kidney disease.