Investigating CSI – Background material Table of Contents I ...

Saliva Evidence
Amylase activity: The most common tests for the identification of saliva are
dependent upon the detection of amylase (1-3). Confirmation of amylase means
that results are consistent with saliva and then consistent with oral sex. A high
level of amylase activity is usually indicative of the presence of saliva. With the
exception of feces, no other body fluid approaches the level of amylase activity in
dried stains. Amylase is a very stable enzyme and can be detected even on
dried articles.
Salivary amylase: As with most enzymes, the function of salivary amylase can
be deduced from the name. "Salivary" refers to the fact that it is found in saliva.
The suffix "ase" is usually added to the end of enzymes as an indication that the
name refers to an enzyme. Amylose is a kind of starch in which glucose
monomers are joined by α(1—4) linkages. α-Amylase (α-1,4-glucan 4glucanohydrolase,
EC 3.2.1.1) is an enzyme that degrades starch to
oligosaccharides and in turn to maltose and glucose by hydrolyzing α-1,4-glucan
bonds. In digestion, the role of α-amylase is primarily the first reaction of this
process, generating oligosaccharides that are then hydrolyzed by other enzymes.
Salivary amylase is very specific in the way it breaks down starch to glucose. It
inserts a water molecule at the α(1—4) linkage closest to the end of the amylose
molecule thus releasing a glucose molecule. We can measure this break down
to determine if salivary amylase is present (see figure below). Positive amylase
tests confirm amylase activity, which is consistent with saliva.
The amylase assay is very sensitive and can detect amylase activity down to a
final concentration of 1 x 10 -4 U/mL (0.001 units). Amylase concentration in
human saliva is 42–59 U/mL (4). One unit (U) is defined as the amount of
enzyme required to liberate 1 mg of maltose from starch in 3 minutes at 20° C, at
pH 6.9.
The mechanism of α-amylase. The enzyme protonates (hydrolizes) the acetal
linkage between the n-sugar residue (sugar on the left) and the rest of the
saccharide chain to yield a free glucose residue to convert to energy.
References
1. Brauner, P., "The Evidence Notwithstanding--A case Report on a Rape,"
Journal of Forensic Sciences, JFSCA, Vol. 37, No.1, Jan. 1992, pp.345-348
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