Hydrogen sulfide is one of three gases involved in biological functions and synthesized in vivo. Like NO and CO, it seems to act as a neuromodulator: it modulates NMDA glutamate receptor function. CBS seems to be the only source of hydrogen sulfide in the brain, whereas the liver synthesizes hydrogen sulfide via cystathionase. In the heart, the third pathway for the hydrogen sulfide synthesis, the 3-mercaptopyruvate pathway is used. Only two diseases characterized by alterations of hydrogen sulfide metabolism have been described: decreased hydrogen sulfide synthesis in the brains of Alzheimer's disease patients and increased hydrogen sulfide synthesis due to the overexpression of CBS in Down syndrome patients.

No abstract

No abstract

Citrulline is a non protein amino acid involved in three important metabolic pathways, the intrahepatic transformation of ammonia to urea, the de novo synthesis of arginine from glutamine in gut and kidney, the nitric oxide synthesis. The two first pathways use the same enzyme activities but are regulated in different way. This review describe these pathways and their regulation in different tissues. In the light of our knowledge we tried to explain the physiological and pathological (inherited or acquired) variations in man.