Protein Carbamylation: Chemistry, Pathophysiological Involvement, and Biomarkers

Abstract

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.

Introduction

In living organisms, proteins are targets of a number of reactions which occur throughout their life span and contribute to the progressive loss of their function and thus to their molecular aging. Among them, nonenzymatic posttranslational modifications (NEPTMs) are major events which participate in this process, inducing alterations of protein structure and functions which explain that an aged protein cannot be functionally considered similar to a native one.

NEPTMs are uncontrolled reactions, which just obey to the mass action law. They are characterized by the spontaneous binding of simple molecules to proteins, followed by more or less complex rearrangements that lead to the formation of a diversity of compounds referred to as posttranslational modification-derived products [1]. These by-products, whenever free or protein bound, are generally irreversibly formed, accumulate in the organism, and are able to exert potent biological actions in tissues. In most cases, they directly (for example, by interference with biological processes) or indirectly (for example, through receptor-mediated interactions) generate deleterious processes. These processes occur during aging due to the time-dependent accumulation of these by-products or are amplified in several chronic diseases, like diabetes mellitus, chronic renal failure, or atherosclerosis, because of their increased production [2].

A typical, well-known modification is nonenzymatic glycation, which is characterized by the binding of simple sugars (typically glucose) or by-products to proteins. This reaction generates in turn oxidative chain reactions, being referred to as glycoxidation, or Maillard reaction in the food industry [1]. It has been shown for many years that posttranslational modification-derived products generated by glycation and glycoxidation reactions, referred to as “advanced glycation end products,” accumulated in skin with age and could reflect the addition of metabolic stresses encountered by the organism throughout life span [3], [4]. Their evaluation by noninvasive techniques like skin autofluorescence, which reflects in part dermal advanced glycation end product content, has brought up the concept of “metabolic memory” of the organism [5], [6]. Indeed, long-lived proteins are preferential targets of NEPTMs, which is typically the case of extracellular matrix proteins like collagens or elastin, which have life spans of several decades [7]. NEPTM-generated modifications lead not only to alterations in the extracellular matrix architecture but also to various side reactions mainly due to inflammatory processes triggered by altered interactions with resident cells.

However, all types of proteins, whatever their location, structure, or life span, are targets of these modifications. NEPTMs have been demonstrated to inhibit biological properties of many biologically active peptides or proteins, such as enzymes or hormones, as reviewed elsewhere [1], [8]. In some cases, enzyme modification could be considered a significant hallmark in cell biology: for example, the decreased activity of mitochondrial glutamate dehydrogenase has been correlated to aging in rat liver extracts [9].

Finally, the structural changes brought by NEPTMs may generate new epitopes making the modified protein recognized as a stranger by the organism leading to the formation of autoantibodies [10].

Carbamylation, which corresponds to the binding of isocyanic acid to proteins, is an NEPTM described some decades ago in vivo and well known in protein chemistry, but its involvement in human pathophysiology and its responsibility in the progression of various diseases have only recently been highlighted [11]. The aim of this chapter is to review the characteristics of carbamylation, its effect on proteins, its involvement in aging and disease, and the interest of its evaluation in clinical chemistry.