Galactosyl‐sucrose metabolism and UDP‐galactose pyrophosphorylase from Cucumis melo L. fruit

Abstract

In muskmelon (Cucumis melo L.), sink tissues receive stachyose, raffinose and sucrose through phloem translocation of carbohydrates that are formed as products of leaf photosynthesis. Melon fruits accumulate sucrose massively during the final stages of maturation. This sucrose is derived partially from the catabolism of raffinose saccharides. Rapid galactose metabolism is required, because liberation of free galactose is the first step in the metabolic utilization of the raffinose sugars. The current study demonstrates that the enzyme UDP‐glucose‐hexose‐1‐P uridylyltransferase (EC 2.7.7.12), the central enzyme in the classical Lelior pathway, is not the central enzyme in galactose metabolism in muskmelon fruit. Rather, a broad substrate specificity UDP‐galactose pyrophosphorylase (PPase) serves the same functional role. This enzyme accepts either UDP‐galactose or UDP‐glucose as a substrate and is different from a UDP‐glucose PPase with more strict substrate specificity for UDP‐glucose that is also present in melon tissue. UDP‐galactose PPase was purified 113‐fold from melon tissue and was shown to be a 54 kDa (size exclusion chromatography) to 68 kDa (SDS‐PAGE) protein that is enzymatically active as a monomer. We also present evidence that the enzyme likely accepts UDP‐galactose and UDP‐glucose at the same catalytic site. Polyclonal antibodies prepared against this protein reacted with numerous other antigens in melon extracts, apparently as a result of the presence of common antigenic epitopes.