Separation and quantitation of peptides and amino acids in normal human urine

Spurious Elevation of Multiple Urine Amino Acids by Ion-Exchange Chromatography in Patients with Prolidase Deficiency

The enzyme prolidase cleaves dipeptides where the C-terminal amino acid corresponds to proline or hydroxyproline. As a consequence, a deficiency of this enzyme leads to accumulation of these dipeptides, which correspondingly are found to be elevated in urine. In fact, the absence of dipeptiduria is sufficient to rule out a diagnosis of prolidase deficiency. However, given the fact that these dipeptides elute at the same position as more common amino acids, the analyzer's software will instead call an elevation of these corresponding amino acids. Thus, an elevation of glycylproline, aspartylproline, glutamylproline, threonylproline and serylproline, valylproline, leucylproline, isoleucylproline, alanylproline, phenylalanylproline, and lysylproline will instead be interpreted as an elevation of leucine, citrulline, methionine, isoleucine, beta-aminoisobutyric acid, gamma-aminobutyric acid, ethanolamine, tyrosine, histidine, and anserine/carnosine, respectively. This particular profile of elevated amino acids, however, can easily be overlooked. We hope that the recognition of this characteristic pattern of falsely elevated urinary amino acids will aid in the recognition of prolidase deficiency.

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.

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

Background: Biuret assays for total protein measurement are considered to react with all peptides longer than 2 residues. Some studies using biuret assays of urine suggest that small peptides generally are more abundant than proteins in urine, but it is not clear whether this is a problem of assay specificity.

Methods: We analyzed the specificity and kinetics of a biuret reaction for solutions of amino acids, organic compounds, peptides, proteins, and ultrafiltered urine specimens and compared the results with standard clinical assays for protein measurement.

Results: The biuret assay cross-reacted with several amino acids, dipeptides, and other organic compounds able to form 5- or 6-member ring chelation complexes with copper. Reactions with amino acids and dipeptides had higher absorbance maxima (blue color) than with larger peptides and proteins (purple). Compounds forming potential 4-, 7-, 8-, or 9-member ring complexes with copper had low reactivity. Amino acid amides, dipeptides, and longer peptides had substantial reactivity, except those containing proline. Proteins and polypeptides had similar biuret reactivities per peptide bond, but reaction kinetics were slower for proteins than peptides. Urine specimens ultrafiltered through 3-kDa–cutoff membranes had substantial biuret reactivity, but absorbance maxima were consistent with cross-reactive amino acids rather than peptides.

Conclusions: Many compounds, including amino acids, amino acid derivatives, and dipeptides, cross-react in biuret assays. Our studies improve understanding of the specificity of endpoint and kinetic biuret assays widely used in clinical laboratories. Amino acids, urea, and creatinine contribute to overestimation of urinary peptide content by biuret assays.

Detection of urinary albumin

Microalbuminuria is an important clinical marker in patients with diabetes and cardiovascular disease. The concentration of albumin in urine has traditionally been measured by semiquantitative dipsticks or by various quantitative immunochemical methods such as immunonephelometry, immunoturbidimetry, and radioimmunoassay. However, until recently, urinary albumin not reabsorbed by proximal tubular cells was assumed to be excreted intact. Studies have now revealed that the nature of urinary albumin is complex and is excreted as a mixture of intact albumin, albumin-derived peptides that are not detected by routine dipstick and antibody-based tests, and a species of intact albumin (immunounreactive albumin), also not detected by dipstick and antibody-based tests. A new test, Accumin, based on high-performance liquid chromatography analysis, is able to detect all the immunoreactive intact albumin and immunounreactive intact albumin (total intact albumin) in urine. The advantage in the use of Accumin over a conventional dipstick test or antibody-based laboratory method for detecting microalbuminuria is that false negatives are reduced and a relatively earlier diagnosis of incipient kidney disease can be achieved. The introduction of Accumin has, therefore, highlighted the need for a global standard in the detection and measurement of microalbuminuria. By detecting all of the immunoreactive and immunounreactive intact albumin in urine, Accumin has virtually invalidated the use of dye and immunologically-based dipstick tests and immunologically-based laboratory methods in screening for microalbuminuria in diabetic patients and in identifying microalbuminuria as a risk factor for cardiovascular disease.

Dipeptide analysis in human urine

Fractions of dipeptides, obtained from human urine by a combination of cation-exchange chromatography, ligand-exchange chromatography and reversed-phase chromatography, were transformed into their N-heptafluorobutyryl methyl ester derivatives and then subjected to capillary gas chromatography. The profiles obtained indicate the presence of many dipeptides in human urine. For the first time, alpha-Asp-Hyp, Pro-Phe and gamma-Glu-Phe were detected in the urine of healthy individuals.