Identification of the primary targets of carbamylation in bovine lens proteins by mass spectrometry

Protein Carbamylation in Chronic Kidney Disease and Dialysis

Protein carbamylation is a nonenzymatic posttranslational protein modification that can be driven, in part, by exposure to urea's dissociation product, cyanate. In humans, when kidney function is impaired and urea accumulates, systemic protein carbamylation levels increase. Additional mediators of protein carbamylation have been identified including inflammation, diet, smoking, circulating free amino acid levels, and environmental exposures. Carbamylation reactions on proteins are capable of irreversibly changing protein charge, structure, and function, resulting in pathologic molecular and cellular responses. Carbamylation has been mechanistically linked to the biochemical pathways implicated in atherosclerosis, dysfunctional erythropoiesis, kidney fibrosis, autoimmunity, and other pathological domains highly relevant to patients with chronic kidney disease. In this review, we describe the biochemical impact of carbamylation on human proteins, the mechanistic role carbamylation can have on clinical outcomes in kidney disease, the clinical association studies of carbamylation in chronic kidney disease, including patients on dialysis, and the promise of therapies aimed at reducing carbamylation burden in this vulnerable patient population.

Protein Carbamylation: Chemistry, Pathophysiological Involvement, and Biomarkers

Protein carbamylation refers to a nonenzymatic modification, which consists in the binding of isocyanic acid on protein functional groups. This reaction is responsible for the alteration in structural and functional properties of proteins, which participate in their molecular aging. Protein molecular aging is now considered a molecular substratum for the development of chronic and inflammatory diseases, including atherosclerosis, chronic kidney disease, or rheumatoid arthritis. As a consequence, carbamylation-derived products have been proposed as interesting biomarkers in various pathological contexts and appropriate analytical methods have been developed for their quantification in biological fluids. The purpose of this review is (i) to describe the biochemical bases of the carbamylation reaction, (ii) to explain how it contributes to protein molecular aging, (iii) to provide evidence of its involvement in aging and chronic diseases, and (iv) to list the available biomarkers of carbamylation process and the related analytical methods.

Protein carbamylation in kidney disease: pathogenesis and clinical implications

Carbamylation describes a nonenzymatic posttranslational protein modification mediated by cyanate, a dissociation product of urea. When kidney function declines and urea accumulates, the burden of carbamylation naturally increases. Free amino acids may protect proteins from carbamylation, and protein carbamylation has been shown to increase in uremic patients with amino acid deficiencies. Carbamylation reactions are capable of altering the structure and functional properties of certain proteins and have been implicated directly in the underlying mechanisms of various disease conditions. A broad range of studies has demonstrated how the irreversible binding of urea-derived cyanate to proteins in the human body causes inappropriate cellular responses leading to adverse outcomes such as accelerated atherosclerosis and inflammation. Given carbamylation's relationship to urea and the evidence that it contributes to disease pathogenesis, measurements of carbamylated proteins may serve as useful quantitative biomarkers of time-averaged urea concentrations while also offering risk assessment in patients with kidney disease. Moreover, the link between carbamylated proteins and disease pathophysiology creates an enticing therapeutic target for reducing the rate of carbamylation. This article reviews the biochemistry of the carbamylation reaction, its role in specific diseases, and the potential diagnostic and therapeutic implications of these findings based on recent advances.

Defective protein folding and aggregation as the basis of neurodegenerative diseases: the darker aspect of proteins

The ability of a polypeptide to fold into a unique, functional, and three-dimensional structure depends on the intrinsic properties of the amino acid sequence, function of the molecular chaperones, proteins, and enzymes. Every polypeptide has a finite tendency to misfold and this forms the darker side of the protein world. Partially folded and misfolded proteins that escape the cellular quality control mechanism have the high tendency to form inter-molecular hydrogen bonding between the same protein molecules resulting in aggregation. This review summarizes the underlying and universal mechanism of protein folding. It also deals with the factors responsible for protein misfolding and aggregation. This article describes some of the consequences of such behavior particularly in the context of neurodegenerative conformational diseases such as Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis and other non-neurodegenerative conformational diseases such as cancer and cystic fibrosis etc. This will encourage a more proactive approach to the early diagnosis of conformational diseases and nutritional counseling for patients.

Determination of dideoxyosone precursors of AGEs in human lens proteins

Dideoxyosones (DDOs) are intermediates in the synthesis of advanced glycation endproducts (AGEs), such as pentosidine and glucosepane. Although the formation of pentosidine and glucosepane in the human lens has been firmly established, the formation of DDOs has not been demonstrated. The aim of this study was to develop a reliable method to detect DDOs in lens proteins. A specific DDO trapping agent, biotinyl-diaminobenzene (3,4-diamino-N-(3-[5-(2-oxohexahydro-1H-thieno[3,4-d]imidazol-4-yl)pentanoyl]aminopropyl)benzamide) (BDAB) was added during in vitro protein glycation or during protein extraction from human lenses. In vitro glycated human lens protein showed strong reaction in monomeric and polymeric crosslinked proteins by Western blot and ELISA. Glycation of BSA in the presence of BDAB resulted in covalent binding of BDAB to the protein and inhibited pentosidine formation. Mass spectrometric analysis of lysozyme glycated in the presence of BDAB showed the presence of quinoxalines at lysine residues at positions K1, K33, K96, and K116. The ELISA results indicated that cataractous lens proteins contain significantly higher levels of DDO than non-cataractous lenses (101.9±67.8 vs. 31.7±19.5AU/mg protein, p<0.0001). This study provides first direct evidence of DDO presence in human tissue proteins and establishes that AGE crosslink synthesis in the human lens occurs via DDO intermediates.

Conformational diseases: looking into the eyes

Conformational diseases, a general term comprising more than 40 disorders are caused by the accumulation of unfolded or misfolded proteins. Improper protein folding (misfolding) as well as accrual of unfolded proteins can lead to the formation of disordered (amorphous) or ordered (amyloid fibril) aggregates. The gradual accumulation of protein aggregates and the acceleration of their formation by stress explain the characteristic late or episodic onset of the diseases. The best studied in this group are neurodegenerative diseases and amyloidosis accompanied by the deposition of a specific aggregation-prone proteins or protein fragments and formation of insoluble fibrils. Amyloidogenic protein accumulation often occurs in the brain tissues, e.g. in Alzheimer's disease with the deposition of amyloid-beta and Tau, in scrapie and bovine spongiform encephalopathy with the accumulation of prion protein, in Parkinson's disease with the deposition of alpha-synuclein. Other examples of amyloid proteins are transthyretin, immunoglobulin light chain, gelsolin, etc. In addition to the brain, the accumulation of unfolded or misfolded proteins leading to pathology takes place in a wide variety of organs and tissues, including different parts of the eye. The best studied ocular conformational diseases are cataract in the lens and retinitis pigmentosa in the retina, but accumulation of misfolded proteins also occurs in other parts of the eye causing various disorders. Furthermore, ocular manifestation of systemic amyloidosis often causes the deposition of amyloidogenic proteins in different ocular tissues. Here we present the data regarding naturally unfolded and misfolded proteins in eye tissues, their structure-function relationships, and molecular mechanisms underlying their involvement in diseases. We also summarize the etiology of ocular conformational diseases and discuss approaches to their treatment.

Diabetic cataracts: mechanisms and management

Diabetes mellitus is associated with a 5-fold higher prevalence of cataracts, which remains a major cause of blindness in the world. Typical diabetic cataracts contain cortical and/or posterior subcapsular opacities. Adult onset diabetic cataracts also often contain nuclear opacities. Mechanisms of diabetic cataractogenesis have been studied in less detail than those of other diabetic complications. Both animal and human studies support important contribution of increased aldose reductase activity. Surgical extraction is the only cure of diabetic cataract today. An improved understanding of pathogenetic mechanisms, together with finding effective therapeutic agents, remain highest priority for diabetic cataract-related research and pharmaceutical development.