Spectrophotometric Assay Using o-Phthaldialdehyde for Determination of Proteolysis in Milk and Isolated Milk Proteins

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Abstract
A rapid, sensitive and convenient spectrophotometric assay was developed and characterized for measurement of proteolysis of milk proteins in buffered solutions or in milk. .alpha.-Amino groups released by hydrolysis react with o-phthaldialdehyde and .beta.-mercaptoethanol to form an adduct that absorbs strongly at 340 nm. The absorptivity is similar for all .alpha.-amino groups. The absorptivity of the adduct with .alpha.- and .epsilon.-amino groups of proteins is also similar and unaffected by local environment when proteins are denatured in sodium dodecyl sulfate. Background is constant for a particular sample and .alpha.-amino groups released by proteolysis can be quantitated accurately. Inclusion of sodium dodecyl sulfate in the assay provides a convenient way to terminate proteolysis and to insure full exposure and complete reaction of amino groups. Because all hydrolytic products are assayed, the method is more accurate than procedures that depend upon properties of aromatic residues (Hull and Lowry methods). The o-phthaldialdehyde spectrophotometric assay is more rapid and convenient than methods using ninhydrin, 2,4,6-trinitrobenzenesulfonic acid or fluorescamine. The assay is especially useful for measuring proteolysis in milk from microbial culture organisms such as Streptococcus lactis C2. Because trichloroacetic acid filtrates are used, the method should be adaptable to other dairy products.