Estimation of Lactose Interference in Vaccines and a Proposal of Methodological Adjustment of Total Protein Determination by the Lowry Method

Abstract

For national regulatory testing in Japan, the Lowry method is used for the determination of total protein content in vaccines. However, many substances are known to interfere with the Lowry method, rendering accurate estimation of protein content difficult. To accurately determine the total protein content in vaccines, it is necessary to identify the major interfering substances and improve the methodology for removing such substances. This study examined the effects of high levels of lactose with low levels of protein in freeze-dried, cell culture-derived Japanese encephalitis vaccine (inactivated). Lactose was selected because it is a reducing sugar that is expected to interfere with the Lowry method. Our results revealed that concentrations of ≥0.1 mg/mL lactose interfered with the Lowry assays and resulted in overestimation of the protein content in a lactose concentration-dependent manner. On the other hand, our results demonstrated that it is important for the residual volume to be ≤0.05 mL after trichloroacetic acid precipitation in order to avoid the effects of lactose. Thus, the method presented here is useful for accurate protein determination by the Lowry method, even when it is used for determining low levels of protein in vaccines containing interfering substances. In this study, we have reported a methodological adjustment that allows accurate estimation of protein content for national regulatory testing, when the vaccine contains interfering substances.