<prompt> Nitric oxide is synthesised from:CysteineCystineMethionineArginine  <response>: NO is synthesized from l-arginine by the enzyme nitric oxide synthase NOS. There are three different types of NOS: endothelial eNOS, neuronal nNOS, and inducible iNOS. eNOS and nNOS are constitutively expressed at low levels and can be activated rapidly by an increase in cytoplasmic Ca2. iNOS, in contrast, is induced when macrophages and other cells are activated by cytokines e.g., TNF, IFN-g or microbial products. Ref: Robbins 8th edition, Chapter 2. 