Cross-Reactivity of Amino Acids and Other Compounds in the Biuret Reaction: Interference with Urinary Peptide Measurements

Comparative effects of high pressure processing and heat treatment on in vitro digestibility of pea protein and starch

The effects of high-pressure processing (HPP) and heat treatment on the digestibility of protein and starch in pea protein concentrate (PPC) were investigated. Samples of PPC with 5% (5 P) and 15% (15 P) protein were treated by HPP (600 MPa/5 °C/4 min) or heat (95 °C/15 min) and their in vitro static and dynamic digestibility were compared to untreated controls. HPP-treated PPC underwent a greater degree of proteolysis and showed different peptide patterns after static gastric digestion compared to untreated and heat-treated PPC. Differences in protein digestibility among treatments during dynamic digestion were only significant (p < 0.05) during the first 20 min of jejunal, ileal, and total digestion for 5 P, and during the first 60 min of ileal digestion for 15 P. Neither static nor dynamic starch digestibility were dependent on treatment. HPP did not reduce trypsin inhibitor activity, whereas heat treatment reduced it by ~70%. HPP-induced structural modifications of proteins and starch did not affect their overall in vitro digestibility but enhanced gastric proteolysis.

Development of a protein assay with copper chelator chromeazurol B, based on the biuret reaction

This study aimed to provide a novel and highly sensitive protein assay based on the biuret reaction and using chromeazurol B, a metal chelate compound. The method consists of two reagents and an automated analyzer. First, a complex of copper and protein (biuret reaction) is formed. Second, a chelating reagent containing chromeazurol B forms a three-dimensional complex of protein, copper, and chromeazurol B at neutral pH, resulting in highly sensitive coloration. The intra-assay (n = 20) variation for the three levels was 3.54 % or lower at each concentration. Each response with α, β-, and γ-globulin was 103.8 % and 104.3 %, respectively, against albumin. The molar absorption coefficient (ε) of the present method was 2.5 × 10⁵ m²/mol against human albumin, higher than that of the commercially available Lowry method (ε = 8.7 × 10⁴ m²/mol), which is based on the same principle. The correlation test for the pyrogallol method with 30 urine samples showed good performance (r = 0.961). The method described here (the Biuret-based CAB method) is a more sensitive and rapid assay than the Lowry method, and it may also be applied to biological samples because of its similar reactivity towards various proteins.

Catalytic Urea Synthesis from Ammonium Carbamate Using a Copper(II) Complex: A Combined Experimental and Theoretical Study

The synthesis of urea fertilizer is currently the largest CO₂ conversion process by volume in the industry. In this process, ammonium carbamate is an intermediate en route to urea formation. We determined that the tetraammineaquacopper(II) sulfate complex, [Cu(NH₃)₄(OH₂)]SO₄, catalyzed the formation of urea from ammonium carbamate in an aqueous solution. A urea yield of up to 18 ± 6% was obtained at 120 °C after 15 h and in a high-pressure metal reactor. No significant urea formed without the catalyst. The urea product was characterized by Fourier transform infrared (FT-IR), powder X-ray diffraction (PXRD), and quantitative ¹H{¹³C} NMR analyses. The [Cu(NH₃)₄(OH₂)]SO₄ catalyst was then recovered at the end of the reaction in a 29% recovery yield, as verified by FT-IR, PXRD, and quantitative UV-vis spectroscopy. A precipitation method using CO₂ was developed to recover and reuse 66 ± 3% of Cu(II). The catalysis mechanism was investigated by the density functional theory at the B3LYP/6-31G** level with an SMD continuum solvent model. We determined that the [Cu(NH₃)₄]²⁺ complex is likely an effective catalyst structure. The study of the catalysis mechanism suggests that the coordinated carbamate with [Cu(NH₃)₄]²⁺ is likely the starting point of the catalyzed reaction, and carbamic acid can be involved as a transient intermediate that facilitates the removal of an OH group. Our work has paved the way for the rational design of catalysts for urea synthesis from the greenhouse gas CO₂.

Modifying the reactivity of copper (II) by its encapsulation into polydimethylsiloxane: A selective sensor for ephedrine-like compounds

This paper demonstrates that the reactivity of copper (II) can be modified through its entrappment in a polymeric matrix of polydimethylsiloxane (PDMS), which makes possible the reaction into the support instead of in solution. Amino-containing compounds such as amino acids, proteins and sugars, which react with Cu (II) in solution, do not react inside the polymer. As a prove of concept, a highly specific Cu (II) PDMS-based sensor for ephedrines has been developed in this work. When the sensors are put into contact with solutions of these drugs under basic conditions, a change in their color from pale green to purple is observed. This change enables the visual identification of ephedrine (Eph) in a few min, as well as its quantification using both reflectance diffuse measurements of the sensors and color intensities of their digitalized images. The sensors show suitable analytical performance for Eph-like compounds, and provide limits of detection (LODs) of 0.3-1.0 mg, and relative standard deviations (RDSs) < 10%. The method has been applied to both the qualitative and quantitative analysis of different types of liquid and solid samples (intravenous injection solution of Eph, dietary supplements and illicit drug-street samples) without the need of any special sample treatment.

The highly abundant urinary metabolite urobilin interferes with the bicinchoninic acid assay

Estimation of total protein concentration is an essential step in any protein- or peptide-centric analysis pipeline. This study demonstrates that urobilin, a breakdown product of heme and a major constituent of urine, interferes considerably with the bicinchoninic acid (BCA) assay. This interference is probably due to the propensity of urobilin to reduce cupric ions (Cu(2+)) to cuprous ions (Cu(1+)), thus mimicking the reduction of copper by proteins, which the assay was designed to do. In addition, it is demonstrated that the Bradford assay is more resistant to the influence of urobilin and other small molecules. As such, urobilin has a strong confounding effect on the estimate of total protein concentrations obtained by BCA assay and thus this assay should not be used for urinary protein quantification. It is recommended that the Bradford assay be used instead.

A miniaturized immunoassay platform to measure neutrophil gelatinase-associated lipocalin (NGAL) for diagnosis of acute kidney injury

In this study, we describe the development and evaluation of a slide-based immunoassay platform for the detection of neutrophil gelatinase associated-lipocalin (NGAL) in plasma and urine samples. The capture NGAL antibody was immobilized onto a microscope slide before an analysis of NGAL based on a sandwich immunoassay was further carried out. This assay system exhibited linearity between 50 to 1000 ng/ml of NGAL. The coefficients of variability (CVs) indicated good reproducibility and repeatability of the system. The levels of plasma NGAL measured by the slide-based system were highly correlated with those of ELISA, while this system over-predicted urine NGAL.

The dynamic range problem in the analysis of the plasma proteome

One of the greatest challenges in analyzing the plasma proteome is the wide range of concentration of different proteins. The current study examines the range of protein concentration for 18 proteins measured over a year in a clinical laboratory to provide data on pathological extremes in protein concentrations. The complete measured range, from upper limits for albumin to lowest values for thyroid-stimulating hormone (TSH), represented more than 10 logs of molar abundance. A number of plasma proteins measured in the clinical laboratory varied over a concentration range spanning more than 4 logs, and limits of detection of clinical assays were inadequate to assess full concentration ranges of several proteins. Considering reported values from studies using higher sensitivity assays suggest that plasma concentrations of prostate-specific antigen (PSA), human chorionic gonadotropin (hCG), and cardiac troponin I vary by more than 7 logs. All of the plasma proteins measured in the present study represent secretory proteins or highly expressed components of specific tissues. Thus, the dynamic range for these components is likely to greatly underestimate the total range of protein concentration in the plasma proteome.

Current issues in measurement and reporting of urinary albumin excretion

BACKGROUND

Urinary excretion of albumin indicates kidney damage and is recognized as a risk factor for progression of kidney disease and cardiovascular disease. The role of urinary albumin measurements has focused attention on the clinical need for accurate and clearly reported results. The National Kidney Disease Education Program and the IFCC convened a conference to assess the current state of preanalytical, analytical, and postanalytical issues affecting urine albumin measurements and to identify areas needing improvement.

CONTENT

The chemistry of albumin in urine is incompletely understood. Current guidelines recommend the use of the albumin/creatinine ratio (ACR) as a surrogate for the error-prone collection of timed urine samples. Although ACR results are affected by patient preparation and time of day of sample collection, neither is standardized. Considerable intermethod differences have been reported for both albumin and creatinine measurement, but trueness is unknown because there are no reference measurement procedures for albumin and no reference materials for either analyte in urine. The recommended reference intervals for the ACR do not take into account the large intergroup differences in creatinine excretion (e.g., related to differences in age, sex, and ethnicity) nor the continuous increase in risk related to albumin excretion.

DISCUSSION

Clinical needs have been identified for standardization of (a) urine collection methods, (b) urine albumin and creatinine measurements based on a complete reference system, (c) reporting of test results, and (d) reference intervals for the ACR.

Disease-dependent mechanisms of albuminuria

The mechanism of albuminuria is perhaps one of the most complex yet important questions in renal physiology today. Recent studies have directly demonstrated that the normal glomerulus filters substantial amounts of albumin and that charge selectivity plays little or no role in preventing this process. This filtered albumin is then processed by proximal tubular cells by two distinct pathways; dysfunction in either one of these pathways gives rise to discrete forms of albuminuria. Most of the filtered albumin is returned to the peritubular blood supply by a retrieval pathway. Albuminuria in the nephrotic range would arise from retrieval pathway dysfunction. The small quantities of filtered albumin that are not retrieved undergo obligatory lysosomal degradation before urinary excretion as small peptide fragments. This degradation pathway is sensitive to metabolic factors responsible for hypertrophy and fibrosis, particularly molecules such as angiotensin II and transforming growth factor-beta1, whose production is stimulated by hyperglycemic and hypertensive environments. Dysfunction in this degradation pathway leads to albuminuria below the nephrotic range. These new insights into albumin filtration and processing argue for a reassessment of the role of podocytes and the slit diaphragm as major direct determinants governing albuminuria, provide information on how glomerular morphology and "tubular" albuminuria may be interrelated, and offer a new rationale for drug development.

Reactivity of Urinary Albumin (Microalbumin) Assays with Fragmented or Modified Albumin

Background: Controversy exists regarding occurrence and measurement of structural variants of albumin in urine. In this study, we examined cross-reactivity of in vitro modified albumins in assays for urine albumin (microalbumin).

Methods: We analyzed albumin modified by reagents, trypsin, or physical treatments or differing in primary sequence (animal albumins) with an immunoturbidimetric assay (Beckman LX20) using goat antiserum and a competitive immunoassay (Siemens Immulite) using a monoclonal antibody. We assessed occurrence of albumin fragments in urine by use of Western blotting of 24 specimens.

Results: Chemical modification, modest sequence substitution (gorilla albumin), or cleavage of albumin by cyanogen bromide (CNBr) had little effect on reactivity in the LX20 assay. Albumin extensively cleaved with trypsin retained partial reactivity. The Immulite assay generally was affected more severely by albumin modifications and sequence changes. Western blots of fresh urine specimens or specimens stored at −80 °C showed little albumin fragmentation, but some specimens stored for 3 years at −20 °C had extensively fragmented albumin that was detected by the LX20 but not the Immulite assay.

Conclusions: Nearly equivalent reactivity of intact albumin and CNBr fragments in the immunoturbidimetric assay indicates reactivity of antibodies with multiple epitopes throughout albumin. Therefore, it is difficult to abolish reactivity of albumin in this type of urine albumin assay. Differential sensitivity of 2 assays to albumin modification identifies a potential source of assay nonequivalence in measuring urinary albumin, particularly for specimens stored at −20 °C.

Diagnostic potential for urinary proteomics

Urine represents a modified ultrafiltrate of plasma, with protein concentrations typically approximately 1000-fold lower than plasma. Urine’s low protein concentration might suggest it to be a less promising diagnostic specimen than plasma. However, urine can be obtained noninvasively and tests of many urinary proteins are well-established in clinical practice. Proteomic technologies expand opportunities to analyze urinary proteins, identifying more than 1000 proteins and peptides in urine. Urine offers a sampling of most plasma proteins, with increased proportions of low-molecular-weight protein and peptide components. Urine also offers enriched sampling of proteins released along the urinary tract. Although urine presents some challenges as a diagnostic specimen, its diverse range of potential markers offers great potential for diagnosis of both systemic and kidney diseases. Examples of clinical situations where this may be of value are for more sensitive detection of kidney transplant rejection or of renal toxicity of medications.