Inhibition of Lactose Dehydrogenase (rLDH) Enzyme Activity by Ascorbic Acid 6-Butyrate (AAB): Comparisons to Ascorbic Acid (AA), Ascorbic Acid 6-Palmitate (AAP), and Ascorbic Acid 6-Stearate (AAS) Inhibitions

Ascorbic Acid (AA) is a natural antioxidant. In vitro studies have shown it to inhibit the activity of the glycolytic enzyme r-Lactose Dehydrogenase (rLDH) from rabbit muscle. While AA has been found to be unstable under physiological conditions, its fatty acid derivatives are stable and have been used in cancer chemotherapy studies. The goal of this research was to study the inhibition properties of fatty acid derivatives of AA, especially Ascorbic Acid-6- Butyrate (AAB) on rLDH. Our general hypothesis was that greater lipophilic character of the AA fatty acid chains enhances hydrophobic interactions with the rLDH enzyme and increases destabilization of its active configurations. Ascorbic Acid-6-Butyrate (AAB) was synthesized by heating AA with Butyric Acid in the presence of Sulfuric acid as a catalyst. In vitro inhibition studies were conducted on r-LDH (41.57 eu) with AAB. The IC₅₀ value was determined and compared to those found for AA, Ascorbic Acid-6- Palmitate (AAP), and Ascorbic Acid-6- Stearate (AAS). Our results showed that Ascorbic Acid Butyrate is seventy-eight times more inhibitory than Ascorbic Acid, and three times more inhibitory than Ascorbic Acid-6- Palmitate and Ascorbic Acid-6- Stearate. Under our experimental conditions, methyl and ethyl butyrate, palmitates, and stearates showed no inhibition. Based on our findings we conclude that the lipophilic character of the side chains in the Ascorbic Acid derivatives is important but not the exclusive factor responsible for the destabilization of rLDH and the observed inhibitions.