A recent In Translation article by Kalim et al in AJKD reviews the clinical relevance of protein cabamylation in kidney disease. Carbamylation refers to the post-translational modification of proteins (other examples being phosphorylation and glycation) that occurs when adding a carbamoyl moiety (-CONH2) to amino acids. It is induced by exposure to urea and its by-product cyanate, both of which are present in high levels in patients with kidney dysfunction. Cassava root contains high levels of cyanate and also contributes to systemic carbamylation when consumed. These modifications may alter protein structure and function leading to pathologic sequelae. Free amino acids compete with proteins as sites of carbamylation so that amino acid deficiency exacerbates the process. This has prompted a clinical trial into amino acid therapy in ESRD. The authors of this review describe a case of a well-dialyzed hemodialysis patient with cardiovascular disease and a significantly elevated carbamylated albumin level. They suggest that protein cabamylation could potentially explain some of the excess cardiovascular risk burden associated with CKD, which is not fully accounted for by anemia, hypertension, and vascular calcification.
Carbamylation largely induces its effects via neutralization of positively charged lysine amino acids, which may alter protein structures. Examples given include inactivation of glutamate dehydrogenase, altering susceptibility of type I collagen to collagenases, and erythropoietin which loses it erythropoietic and angiogenic effects when carbamylated. Furthermore, there is evidence that carbamylated albumin may be nephrotoxic. Effects on other systems include:
Cardiovascular Disease: Cardiovascular disease may be aggravated by several mechanisms. Carbamylation of low-density lipoprotein (and likely high-density ones, too) increases its atherogenicity and ability to make foam cells with macrophages. It may also directly cause proliferation of vascular smooth muscle cells. Moreover, cyanate may be directly toxic to endothelial cells.
Chronic Kidney Disease: The ubiquitous nature and free diffusability of urea means that most if not all tissues in the body are exposed to carbamylation, which occurs to a higher degree with reduced GFR. Accelerated carbamylation could potentially exaggerate the progression of the kidney disease, with evidence of glomerular mesangial cells becoming fibrotic in response to carbamylated proteins.
Biomarkers: Carbamylated proteins may serve as time-averaged indicators of ‘urea-burden’ in patients with kidney disease in a similar fashion to glycated hemoglobin for glycemic control in diabetics. Hemoglobin is also carbamylated, which correlates with kidney disease and perhaps lower dialysis clearance, in at least one study. It should be noted that no studies have looked at carbamylated hemoglobin with respect to patient outcome, and the reduced red cell survival in ESRD may limit its applicability (much like glycated hemoglobin in this population). The authors suggest a potential use in distinguishing acute from chronic kidney disease using carbamylated hemoglobin, which has been demonstrated in two studies.
Modern technologies allow for quantitative measurement of total protein carbamylation and screening for specific modifications using mass spectrometry and proteomics. These advances have led to observations such as carbamylated protein levels being associated with erythropoietin resistance and predicting mortality risk in ESRD. Global protein carbamylation (a time-averaged assay by definition) appears to be more predictive for mortality than traditional urea-based dialysis dose measurements, such as URR or KT/V .
To summarize, carbamylation induced by urea and its by-product cyanate interferes with protein function and may potentially explain much of the comorbidity associated with poor kidney function. It serves as a unique method to assess time-averaged uremia, potentially serving as an indicator of poor dialysis clearance beyond traditional urea kinetics. It also challenges the belief that urea is an innocent bystander in kidney dysfunction and not itself a uremic toxin.
Dr. Paul Phelan
Royal Infirmary of Edinburgh
AJKD Blog Contributor