|Abstract or Summary
- 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
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
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