XVI International Botanical Congess
Ascorbate biosynthesis in plants has remained unresolved until recently, many steps remain to be elucidated. The ultimate step of the pathway involves the conversion of L-galactono-gamma-lactone (GL) to ascorbic acid, a reaction catalysed by the membrane-bound enzyme, L-galactono-lactone dehydrogenase (GLDHase). The activity of this enzyme is frequently measured by the concomitant reduction of cytomchrome C but the natural oxidising substrate in unknown. Up until now the activity of this enzyme has only been measured in purified mitochondria. A simple system for this enzyme was therefore devised. Sub-cellular fractionation experiments and Western blots revealed that 90% of the GLDHase activity was localised in the mitochondrial inner membrane, the remaining 10% being associated with the microsomal fraction.