A 35-year-old male with a family history of lactose intolerance undergoes investigation for suspected lactase deficiency. Intestinal biopsy samples are obtained and enzyme activity is measured at varying lactose concentrations (0.5 mM to 50 mM). The researcher observes that the enzyme shows reduced Vmax but normal Km compared to healthy controls. Which investigation would be most appropriate to determine whether this represents a quantitative enzyme deficiency or a qualitative defect in the enzyme protein?