Structure and function of peptide mediators::
Size varies from three to several hundred amino acids conventionally molecules of fewer than 50 residues are called peptides large molecules being proteins
Neural and endocrine mediators range in size from 3 to over 200 residues cytokines chemokines and growth factors are generally larger than 100 residues
Most known peptide mediators come from the nervous system and endocrine organs however some are formed in the plasma and many occur at other sites (e.g. vascular endothelium heart cells of the immune system etc) The same peptide may occur in several places and serve different functions
Small peptides and chemokines act mainly on G- protein -coupled receptors and act receptors and act through the same second messenger systems as those used by other mediators cytokines and growth factors generally act throuth tyrosine - kinase -linked membrane receptors
peptides frequently function in the nervous system as co transmitters with other peptides or with non-peptide transmitters
The number of known peptide mediators now greatly exceeds that of non -peptides
KININS
Kinins are a group of potent vasodilator peptides that are formed enzymatically by the actions of kallikreins of kininogenases on protein substrates called kininogens kallikreins are present in plasma and in several tissues including the kidneys pancrease intestine sweat glands and salivary glands they are glycoproteins similar to the enzyme like trypsin elastase etc they circulate in the blood as a precursor prekallikrein which is produced in liver plasma kallikrein can be activated by trypsin hageman factor and possibly kallikrein itself it can convert prorenin to active renin
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