However, evaluation of the PPP by using the ratios of 14CO2 produced from [1-14C]glucose and [6-14C]glucose is not appropriate, and the absence of a difference in yields of 14CO2 does not prove that the PPP is absent or inactive.
14CO2 can be released from [1-14C]glucose by metabolism through the PPP or in the Krebs cycle after glycolytic metabolism to [3-14C]pyruvate. In the presence of unlabeled pyruvate and lactate, which greatly decrease the specific activity of radiolabel entering the Krebs cycle, the 14CO2 released by the Krebs cycle is necessarily negligible. Under our experimental conditions (medium containing pyruvate and lactate), the oxidation of labeled glucose through the Krebs cycle was prevented since no detectable levels of 14CO2 were measured after incubation with [6-14C]glucose. Buy Advair Diskus Online
Therefore, the 14CO2 measured after incubation of spermatozoa with [1-14C]glucose originated exclusively from the PPP and was indicative of a functional PPP. The ratio of the 14Co2 produced from [1-14C]glucose to the 3H2O produced from [5-3H]glucose, which has been used to determine the relative activity of the PPP, was very low. The activity of this pathway would be higher if recycling of ribose 5-phosphate occurred; however, this could not be evaluated by using [1-14C]glucose.