Glucose catabolic pathways in Lentinula edodes determined with radiorespirometry and enzymic analysis

Abstract

Carbohydrate metabolism in different tissues was studied using radiorespirometry and enzymic analysis. The 14 C-1/ 14 C-6 ratios in CO 2 respired from specifically labelled glucose fed to Lentinula edodes tissues grown on a chemically defined medium ranged from 2·5 (vegetative mycelium) to 14·9 (young lamellae). This reflects the relative activity of the pentose phosphate pathway (PPP), very high in basidiome but low in mycelium, the highest ratio being recorded in tissues which are biosynthetically most active (young lamellae), requiring the reducing power of NADPH generated through the PPP. Extensive conversion of 14 C-3, 4-labelled glucose to 14 CO 2 in the mycelium underlined the important role of the Embden-Meyerhof-Parnas pathway (EMP) in that tissue. A 14 C-1/ 14 C-6 ratio of 14·8 for young lamellae grown on woodchips indicates that the growth medium did not influence the pathway used in the basidiome. Ratios for the young pileipellis and stipe were 8·0 and 10·4 respectively. A ratio of 3·6 for Coprinus cinereus gills confirms the comparatively lesser importance of the PPP in this organism. Enzymic determination corroborated the metabolic pattern deduced from radiolabelling. Activity of PPP enzymes (glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) were three times as high in the basidiome compared with the mycelium, highest activity being in the young pileus. EMP enzymes (fructose 1,6-bisphosphate aldolase and glucose 6-phosphate isomerase) were more active in mycelium than in fruit body, and also more active than PPP enzymes within the mycelium itself. Within the basidiome, specific activities of EMP and PPP enzymes were about the same. Enzymic activity in mature pileus of C. cinereus resembled the pattern in the mycelium rather than the basidiome of L. edodes ; EMP enzymes were very much more active than PPP. The positive correlation between high PPP activity and accumulation of large amounts of mannitol in the basidiome is discussed.