Stability of prostaglandin E1 and dinoprostone (prostaglandin E2) under strongly acidic and basic conditions

The stability of prostaglandin E1 and dinoprostone was investigated at the extremes of the pH range (less than or equal to 3 and greater than or equal to 10) in the sequence prostaglandin E leads to prostaglandin A leads to prostaglandin B. The degradation rate is first order with hydrogen-ion and hydroxide-ion concentrations. Separation and analysis of the E prostaglandins were accomplished by TLC and UV spectrophotometry. At the lowest pH values and at elevated or low temperatures, significant amounts of 15-epiprostaglandin E were present. Apparent activation energies for the total dinoprostone loss, calculated from elevated temperature data, were 21 kcal/mole in the strongly acidic region and about 18 kcal/mole at pH 3. Corresponding studies in the alkaline region led to a derived arrhenius activation energy of 15 kcal/mole with the appearance of significant amounts of 8-isoprostaglandin E. This difference in activation energies may reflect the different mechanisms operant at high and low pH values.