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Document<br />

4<br />

Bradykinin Receptor Antagonists<br />

Donald J. Kyle<br />

Scios Nova Inc., Sunnyvale, California<br />

I. Introduction<br />

Page 119<br />

The term “kinins” is generally made in reference to either the nonapeptide bradykinin (Arg1-Pro2-Pro3- Gly4-Phe5-Ser6-Pro7-Phe8-Arg9) or the decapeptide kallidin (Lys1-Arg2-Pro3-Pro4-Gly5-Phe6-Ser7-Pro8- Phe9-Arg10). In rats another kinin, Ile1-Ser2-Arg3-Pro4-Pro5-Gly6-Phe7-Ser8-Pro9-Phe10-Arg11 (T-kinin) is<br />

produced under certain circumstances and binds to the same receptors as bradykinin [1,2]. A schematic<br />

of the human kinin-kallikrein system is shown in Figure 1.<br />

The release of kinins from precursor proteins (known as kininogens) is mediated <strong>by</strong> enzymes called<br />

kininogenases [3–5]. The predominant enzymes responsible are kallikreins, but others, which include<br />

trypsin, plasmin, and some snake venoms, also release kinins. Kininogens are primarily synthesized in<br />

the liver and represent an abundant source of the precursors that are required for kinin generation. These<br />

proteins are produced from alternative splicing of a single gene product and there are two forms: high<br />

molecular weight kininogen (HMWK) and low molecular weight kininogen (LMWK) [6]. Unlike<br />

HMWK, which exists in the circulation as a complex with plasma pre-kallikrein, LMWK circulates<br />

freely.<br />

During immunological reaction, charged surfaces—which may be derived from bacterial<br />

lipopolysaccharide, oligosaccharides, connective tissue proteoglycans, or damaged basement<br />

membranes—facilitate the conversion of factor XII to factor XIIa. Once factor XIIa is present, prekallikrein<br />

can be cleaved to its active form, known as plasma kallikrein. This enzyme acts upon its<br />

preferred substrate, HMWK, to release bradykinin. Plasma kallikrein is further able to convert inactive<br />

factor XII to active XIIa, there<strong>by</strong> participating in a positive<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_119.html [4/5/2004 4:56:06 PM]

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