The reactions of aldehydes with proteins are of nutritional and physiological significance. The nutritive value of food materials can be impaired by a reaction necessitating additional dietary protein, and enzymes have been shown in vitro to be inhibited by reactions with aldehydes. Aldehydes present may arise from the autoxidation of lipid materials. A diversity of carbonyl products can occur. Several autoxidizing food lipid systems have been analyzed by other investigators, and the carbonyls present isolated and identified. In most cases the short chain aldehydes predominated. Reactions between aldehydes and proteins have been studied mainly with formaldehyde. Reactions between the aldehyde and many protein functional groups have been suggested including amino, amide, guanidyl, sulfhydryl and imidazole. The amino groups of the protein have received the most attention. The reactions proposed have included: a. methylolamine formation followed by condensation forming a cross link, or b. Schiff's base formation. The reactions have been found to be pH specific with the pH of maximum reaction depending upon the particular protein used. Possible reactions of dialdehydes with proteins have been little explored. Of interest are the possible reactions of malonaldehyde, one of the dialdehydes occurring in autoxidizing lipid systems. Herein, the action of malonaldehyde has been compared to that of formaldehyde, considered a reactive aldehyde, and to propionaldehyde, its monoaldehyde counterpart. The enzyme lipase was used as both the protein and the system of analysis for following the course of the reaction. Lipase activity was determined by potentiometrically titrating the fatty acids liberated from an olive oil emulsion. The commiercial bovine pancreatic lipase preparation used was found to have an activity optimum near pH 9.0. The relationship of the amount of enzyme present to the amount of fatty acids liberated was determined and found to be linear. Maximum stability of the control enzyme appeared between pH 6.0 and 6.5, while maximum lability in the presence of MA-Na (the sodium salt of the enolic form of malonaldehyde) was also near pH 6.0. Formaldehyde and propionaldehyde, however, both exhibited a non-pH-specific inhibition of lipase. Of the three aldehydes reacted with lipase at pH 6.0 and analyzed at pH 9.0, MA-Na was the most inhibitory. The reaction appeared to be two stage, the first being rapid and a function of the MA-Na concentration. The second stage was slower, and the rate was related linearly to the MA-Na concentration. Methanol-free formaldehyde did not inhibit lipase as effectively as MA-Na. Low concentrations (0.01 M) of formaldehyde had no apparent effect on the enzyme while concentrations above 0.05 M produced only slight changes in the degree of inhibition. Propionaldehyde showed only slight inhibition of lipase activity. Its reaction was two stage, with the second stage paralleling the inhibition seen in the control due to factors other than the aldehyde present. Reactions such as these, especially for the more reactive aldehydes such as malonaldehyde, may occur in vitro and in vivo with other proteins and could be of important nutritional or physiological significance. / Graduation date: 1964

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