Kidneys are the organs filtering waste products from blood, and creating urine and kidney function is frequently measured as the glomerular filtration rate (GFR). Recently, advances in mass methodology have allowed comprehensive studies of metabolomics and its relationship with kidney function [21–25]. Metabolomics studies can identify and quantify all metabolites present in a given sample, covering hundreds to thousands of metabolites. Thus, data such as the heatmap and plots, as well as principal component analysis and random forest analysis, are performed on the entirety of the data set, the plasma-only data set, and the serum-only data set. Major differences in metabolite profiles in the various severities of CKD were observed. A large number of biochemicals increased with the progression of CKD; on the other hand, a small number of biochemicals were reduced. These differences may reveal stage-specific biomarkers of CKD.
In this study, we found a number of metabolites associated with mGFR, and we selected the top 30 metabolites that were strongly related to mGFR by the random forest method,As shown in(Fig. 1–3), Here, we find 10 of the top ranking 15 metabolites substances were concordant between plasm group and serum group .This indicates that the results of serum samples and plasma samples for the determination of metabolomics may be generally consistent;however, still need more studies to verify.
Sekula et al. [26] reported 56 metabolites that replicated as associated with eGFRcr, including 6 metabolites that were consistently strongly correlated with eGFRcr (pseudouridine, c-mannosyltryptophan, N-acetylalanine, erythronate, myo-inositol and N-acetylcarnosine). However, Coresh J et al [16] reported a few candidate novel filtration markers of metabolites in a panel including pseudouridine, acetylthreonine, myoinositol, phenylacetylglutamine and tryptophan and high correlation with mGFR (including all of the above metabolites except N-acetylcarnosine).
In our research, we found that C-glycosyltryptophan (also known as C-mannosyltryptophan) pseudouridine, N-acetylalanine, erythronate, myo-inositol and even N-acetylcarnosine were highly correlated with mGFR, except acetylthreonine. N-acetylcarnosine and pseudouridine showed markedly increasing levels with increased nephropathy in our study, consistent with the results reported in Sekula et al. [26]. Both metabolites can be indicators of protein turnover as N-acetylation of amino acids. Pseudouridine is a derivative of uridine and is a modified nucleoside found in RNA. Interestingly, pseudouridine may be an ideal biomarker ranking in the top 5 among the above studies, meaning it is a stable indicator and nondependent on race.
Additionally, both N6-carbamoylthreonyladenosine and hydroxyasparagine were unique in this study. Hydroxyasparagine, known as β-hydroxyasparagine (beta-hydroxyasparagine), is associated with mGFR and CKD and is a modified asparagine amino acid. However, we know little about this metabolite. It appears in posttranslational modifications of EGF-like domains that can occur in humans and others Eukaryotes. The modified amino acid residue is found in fibrillin-1[27]. C-glycosyltryptophan was identified to be associated with mGFR and CKD, as well as with prospective endpoints of eGFR decline, incident CKD and ESRD[26].
Specifically, a potential negative biomarker of kidney disease is 1,5-anhydroglucitol (1,5-AG). 1,5-AG. In the kidney, 1,5-AG is filtered by the glomerulus, and the majority is reabsorbed in the proximal tubule back to the blood. Glucose, which trended higher in the moderate and severe nephropathy groups than in the normal kidney function group, is a competitive inhibitor of this reabsorption. Thus, if glucose rises, more 1,5-AG is excreted in the urine, lowering blood levels. The level of 1,5-AG was decreased in severe nephropathy compared to the three other groups and in moderate nephropathy compared to mild nephropathy. In a recent study, 1,5-AG may also have prognostic value in relation to cardiovascular events in patients with coronary heart disease[28].
Creatine kinase catalyzes both the transfer of a high-energy phosphate from ATP to creatine and the regeneration of ATP from creatine phosphate and ADP. In solution, creatine slowly and spontaneously cyclizes to creatinine, which is eliminated in the urine and can be used as a marker of kidney function. Creatinine levels increase with increasing nephropathy. The urea cycle is necessary for organisms to detoxify and safely eliminate waste ammonia generated from the catabolism of amino acids. Urea also increases with increasing nephropathy.
In addition to removing waste from the organism, another function of the kidney is to regulate the electrolyte and fluid volume for the body. Thus, with nephropathy, derangements in molecules necessary for osmotic regulation are expected. As seen in Fig. 4, increases in small molecules involved in osmotic regulation, such as inositol metabolism, myo-inositol and chiro-inositol, erythronate, and trimethylamine N-oxide (TMAO), were observed with increasing nephropathy. Other metabolites, such as 3-indoxyl sulfate, phenylacetylglutamine, 1-methylguanidine, and guanidinosuccinate, have been shown to be uremic toxins or metabolites that accumulate during uremia. All three of these metabolites show increasing concentrations with increased nephropathy. Guanidine (G), 1-methylguanidine (MG), and 1,1-dimethylguanidine (DMG) have long been implicated as uremic "toxins." N-acetylalanine-aminopeptidase is a new enzyme found in human erythrocytes. Enzymatic activity has not been found in the cytosolic compartment of highly purified human leucocytes [29]. Its physiological function in erythrocytes is still unknown.
Tryptophan, an essential aromatic amino acid, can feed into a number of processes, including production of the neurotransmitter serotonin, the anti-inflammatory metabolite kynurenine, and downstream of kynurenine, NAD + production. An enzyme that catalyzes the first step in the conversion of tryptophan to kynurenine is tryptophan 2,3-dioxygenase (TDO), which is primarily expressed in the liver. Additionally, changes in tryptophan metabolites can also reflect increasing inflammation: indoleamine 2,3-dioxygenase (IDO), which also catalyzes the conversion of tryptophan to kynurenine, is activated by proinflammatory cytokines (e.g., IFN-γ and TNF-α). While kynurenine is produced in this way by an inflammatory process, it has an anti-inflammatory function, serving as a brake on the immune response. Multiple metabolites in the tryptophan metabolic pathway, including kynurenine, kynurenate, anthranilate, and xanthurenate, were increased in the severe and moderate nephropathy groups, with some metabolites increased in the mild group compared to the normal group, indicative of increased inflammation (data not shown). C-glycosyltryptophan results from a posttranslational modification of tryptophan by linking a sugar by a carbon–carbon bond[30–31]. Certain posttranslational modifications, such as carbamylation, have been associated with different chronic conditions, including CKD[32]. Studies in humans showed consistently increased levels of C-glycosyltryptophan in people with decreased eGFR[33–35]. Moreover, studies of humans and animal models suggested C-glycosyltryptophan to be a good biomarker of kidney function with more favorable properties than serum creatinine[35, 36].
In our study, numerous xenobiotics and metabolized xenobiotics and other metaboletes were observed. Some of note are differences in therapeutic drugs. These drugs, which can have significant systemic effects, are a confounding factor in the data.