ABSTRACT
A technique for electrochemical detection of Trp-, Tyr-, and sulfur-containing peptides, using a two-step potential waveform at a platinum wall-jet electrode, has been developed. The detection is fully compatible with reversed-phase HPLC employing gradients of acetonitrile in water/trifluoroacetic acid. At approximately +1.2 V (first potential) versus Ag/AgCl, Trp-, Tyr-, and Cys-containing peptides are predominantly detected, while at +1.4 - 1.6 V, Met- and (Cys)2-containing peptides are additionally detected. The electrode surface is cleaned by the second potential (+2.0 V). the linearity is at least 2 orders of magnitude. The sensitivity is in the picomole range. By using postcolumn electrochemical conversion, the selectivity toward Met and Cys-containing peptides can be enhanced. Applications are shown for the determination of caseinomacropeptide (6.6 kDa) and a tryptic map of beta-casein.
Subject(s)
Caseins/analysis , Milk/chemistry , Peptide Fragments/analysis , Amino Acid Sequence , Animals , Caseins/chemistry , Cattle , Chromatography, High Pressure Liquid , Electrochemistry , Electrodes , Molecular Sequence Data , Peptide Fragments/chemistry , Peptide Mapping , PlatinumABSTRACT
A rapid method was developed for the simultaneous determination of hippuric and orotic acid in rennet whey by capillary zone electrophoresis using an uncoated capillary utilizing a 0.04 M amino-2-methyl-1,3-propanediol (AMPD)-N,N-bis(2-hydroxyethyl) glycine (BICINE) buffer (pH 8.8) with UV detection at 254 and 280 nm. Whey proteins were removed by ultrafiltration. The method was evaluated for external, internal and standard addition procedures for both peak areas and peak heights. The use of an internal standard (sorbic acid) eliminated injection errors and gave, when applied to peak areas, the same levels for hippuric and orotic acid in those obtained with high-performance liquid chromatography. Relative standard deviations were 1-2%. Peak heights gave erratic results owing to sample matrix effects on peak widths.
Subject(s)
Dairy Products/analysis , Hippurates/analysis , Orotic Acid/analysis , Electrophoresis , Indicators and Reagents , Spectrophotometry, Ultraviolet , UltrafiltrationABSTRACT
Bovine kappa-casein was fractionated at pH 8.0 on DEAE-Sepharose with an NaCl gradient, followed by DEAE-cellulose chromatography using a decreasing pH gradient from pH 6.0 to 4.5. At least ten components could be identified, each differing in N-acetylneuraminic acid (NeuAc) and/or phosphorus content. Two components appeared to be multiply-phosphorylated, but did not contain NeuAc. The possible significance of this finding in relation to the mode of phosphorylation and glycosylation in vivo is discussed. A carbohydrate-free fraction as well as two NeuAc-containing fractions were compared in their substrate behaviour towards the action of the milk-clotting enzyme chymosin at pH 6.6 and 30 degrees C. To this end the trichloroacetic acid-soluble reaction products were analysed by high-performance gel-permeation chromatography. In order of increasing carbohydrate content the kcat. values found ranged from 40 to 25 s-1 and the Km values from 9 to 3 microM; the overall substrate properties of these components as reflected by the kinetic parameter kcat./Km ranged from 5 to 8 microM-1 X S-1. Irreversible polymerization of the carbohydrate-free fraction brought about a more-than-2-fold increase in Km, the kcat. value remaining virtually constant. The kcat./Km found for the cleavage of whole kappa-casein at pH 6.6 was of the same magnitude as the kcat./Km found for the polymerized carbohydrate-free fraction (i.e. about 3 microM-1 X S-1). No indication of substrate inhibition was found for the carbohydrate-free fraction.
