Rennin is a milk coagulant that is an enzyme obtained from the abo-
masum portion of the stomach of suckling mammals. it is most
active at ph 3.8. one part purified rennin will coagulate more than
five million parts of milk. the commercial extract of rennin is termed
rennet. it is used to coagulate milk in making cheese, junket, and
custard. see rennet.
定義
An enzyme secreted by cellslining the stomach in mammals thatis responsible for clotting milk. Itacts on a soluble milk protein (caseinogen),which it converts to theinsoluble form casein. This ensuresthat milk remains in the stomachlong enough to be acted on by protein-digesting enzymes.
生物学の機能
Renin is an enzyme that is synthesized and stored in the
renal juxtaglomerular apparatus and that catalyzes the
formation of a decapeptide, angiotensin I, from a
plasma protein substrate. Renin has a narrow substrate
specificity that is limited to a single peptide bond in angiotensinogen,
a precursor of angiotensin I. Renin is
considered to control the rate-limiting step in the ultimate
production of angiotensin II. Control of renin secretion
by the juxtaglomerular apparatus is important
in determining the plasma renin concentration.
Three generally accepted mechanisms are involved
in the regulation of renin secretion. The first
depends on renal afferent arterioles that act as stretch
receptors or baroreceptors. Increased intravascular
pressure and increased volume in the afferent arteriole
inhibits the release of renin. The second mechanism is
the result of changes in the amount of filtered sodium
that reaches the macula densa of the distal tubule.
Plasma renin activity correlates inversely with dietary
sodium intake. The third renin secretory control mechanism
is neurogenic and involves the dense sympathetic
innervation of the juxtaglomerular cells in the afferent
arteriole; renin release is increased following activation
of 1-adrenoceptors by the neurotransmitter norepinephrine.
Angiotensin II, the primary end product of the
renin–angiotensin system, acts on the juxtaglomerular
cells to inhibit the release of renin; this process is therefore
a negative feedback mechanism. The half-life of
renin in the circulation is 10 to 30 minutes, with inactivation
occurring primarily in the liver. Small amounts of
renin are eliminated by the kidneys. Pure human renin has been used to develop specific inhibitors of the enzyme.
Low-molecular-weight orally effective renin inhibitors
are under development.
