Glucose-signalled inhibition of cysteine-proteases involved in nitrate reductase degradation in oat leaf segments

Regulation of nitrate reductase (NR; EC 1.6.6.1) breakdown, measured as loss of maximal activity (MNRA), was studied in leaf segments of 7-day-old oat plants in the light for up to 4 h. In segments floating on 1 mM tungstate, NR lost more than 40% of its initial maximal activity. Cycloheximide, high (300 mM) glucose (Glc) and inhibitors of cysteine proteases stabilized NR in situ , suggesting that MNRA decrease was due to the hydrolysis of NR by a short-lived, glucose-modulated cysteine protease. Loss of MNRA was accelerated by cantharidin (CTHR) and inhibited by staurosporine, suggesting that NR breakdown required continuous phosphorylation. High glucose inhibited any further MNRA decrease when supplied after a 30-min pretreatment with CTHR, suggesting that a phosphorylated protein was its target. Isoosmolar polyethylene glycol also stabilized NR but not in the presence of CTHR. Low (30 mM) Glc stabilized NR only in the presence of Ca²⁺, and CTHR inhibited its effect. EGTA and LaCl₃ completely arrested the effects of both high- and low- Glc. Like low D-Glc, low L-Glc (glucose analog not transported) inhibited NR breakdown in the presence of Ca²⁺, but at high concentration only 2-deoxyglucose, that is phosphorylated but not further metabolized, and glucose-6P were effective in the presence of CTHR, suggesting that receptors for high- and low- Glc were located in different cell compartments. It is proposed that high- and low- Glc trigger different signalling pathways, with calcium as a common upstream secondary messenger and protein kinases and protein phosphatases being downstream components in the cascade of reactions that modulates NR proteolysis.     

Hemostasis and ageing

On March 19, 2008 a Symposium on Pathophysiology of Ageing and Age-Related Diseases was held in Palermo, Italy. The lecture of D. Mari on Hemostasis and ageing is summarized herein. Physiological ageing is associated with increased plasma levels of many proteins of blood coagulation together with fibrinolysis impairment. This may be of great concern in view of the known association between vascular and thromboembolic diseases and ageing. On the other hand, centenarians are characterized by a state of hypercoagulability and possession of several high-risk alleles and well-known atherothrombotic risk markers but this appears to be compatible with longevity and/or health. Parameters considered risk factors for atherosclerotic vascular diseases in young people may lose their biological significance in advanced age and assume a different role.