Pineapple organic acid metabolism and accumulation during fruit development

Developmental changes in pineapple (Ananas Comosus (L.) Merrill) fruit acidity was determined for a ‘Smooth Cayenne’ high acid clone PRI#36-21 and a low acid clone PRI#63-555. The high acid clone gradually increased in fruit acidity from 1.4 meq/100 ml 6 weeks from flowering, and peaked a week before harvest at ca 10 meq/100 ml. In contrast, the low acid clone increased in acidity 6 to 8 weeks after flowering, peaked 15 weeks after flowering at ca. 9 meq per/100 ml and then sharply declined in 2 weeks to 6 meq/100 ml. The increased in total soluble solids (TSS) of the low acid clone began 6 weeks after flowering and for the high acid clone at 12 weeks after flowering. The increase in titratable fruit acidity (TA) paralleled the changes in the citric acid content of both clones. Citric acid content increased from less than 1 mg/g at 6 weeks after flowering to 6 to 7 mg/g, 9 weeks later. The malic acid concentration in both clones varied between 3 and 5 mg/g and showed no marked changes just before harvest. The developmental changes in fruit potassium were significantly correlated with fruit acidity and fruit total soluble solids in both the high and low acid clones. Developmental changes in acid-related enzymatic activities showed an increase in citrate synthase (EC 4.1.3.7) activity that occurred a week before harvest, coincided with the peak in citric acid in the high acid clone. An increase in aconitase (ACO, EC 4.2.1.3) activity was observed just before harvest as the decline in acidity occurred in the low acid clone. The activities of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), malate dehydrogenase (MDH, EC 1.1.1.37) and malic enzyme (ME, EC 1.1.1.40) did not parallel any changes in fruit acidity. The results indicated that the change in pineapple fruit acidity during development was due to changes in citric acid content. The major difference in acid accumulation occurred in the low acid clone just before harvest when acidity declined by one-third. The activities of citrate synthase and aconitase possibly played a major role in pineapple fruit acidity changes.     

Effect of mitochondrial aconitase activity on energy synthesis during aging

AIM：To investigate the effect of mitochondrial aconitase (ACO2) on energy synthesis during aging in male SD rats and D-galactose-induced cell aging model. METHODS：D-galactose at concentration of 55 mmol/L was used to establish MRC-5 cell aging model. Intact mitochondria in the rat brain and MRC-5 cells were isolated by a sucrose density gradient centrifugation. The formation of NADPH was used to represent the ACO2 activity and determined by observation of the absorbance at 340 nm. Fluorescence quantitative PCR and Western blotting were applied to detected ACO2 expression. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were measured by the commercial assay kits. The tissue iron content was detected by ferrozine method. Mitochondrial membrane potential was detected by JC-1 mitochondrial membrane potential detection kit. The content of ATP, ADP and AMP was measured by HPLC analysis, and the energy charge was then calculated by the formula. RESULTS：ACO2 activity and iron content presented age-related decline and increase, respectively, while the expression level of ACO2 kept stable. ACO2 activity significantly declined when the cells were treated with hydrogen peroxide at different concentrations. In the aging cells, SOD activity and ACO2 activity were decreased and MDA content was increased significantly, while the expression level of ACO2 was unchanged. During aging, mitochondrial membrane potential, ATP synthesis and energy charge presented significant reduction. CONCLUSION：The activity of ACO2, which is sensitive to oxidative stress, declines during aging, and may affect the efficiency of the Krebs cycle. At the same time, mitochondrial membrane potential decreases, thus reducing the energy synthesis in mitochondria.