X

X RAY ANGIOGRAPHY

an invasive radiographic technique where a radio-opaque dye is injected into a blood vessel for the purpose of characterizing its anatomy on an X-ray. This technique is used to image arteries in the brain, heart, kidneys, gastrointestinal tract, aorta, neck (carotids), chest, limbs and pulmonary circuit

X RAY VENOGRAPHY

subset of x-ray angiography; venograms deal strictly with the veins as opposed to the arteries, most commonly the veins in your legs, though other sets of veins can be examined

XENOBIOTIC

Synthetic compds foreign to living systems

XENOGRAFT

Transplant from one individual to another individual of a different species

XERODERMA PIGMENTOSUM

The disease keeps sufferers indoors because they do not produce an enzyme that repairs DNA damage induced by UV radiation

XEROSTOMIA

Mouth dryness

XEROSTOMIA

Mouth dryness

XYLOGLUCANASE

Xyloglucan is a major structural polysaccharide in the primary (growing) cell wall of plants. Structurally, xyloglucans consists of a cellulose-like beta-1,4-linked glucose backbone which is frequently substituted with various side chains. The xyloglucans of most dicotyledonous plants, some monocotyledons and gymnosperms are highly branched polysaccharides in which approx. 75% of the glucose residues in the backbone bear a glycosyl side chain at O-6. The glycosyl residue that is directly attached to the branched glucose residue is invariably alfa-D-xylose. Up to 50% of the side chains in the xyloglucans contain more than one residue due to the presence of beta-D-galactose or alfa-L-fucose-(1-2)-beta-D-galactose moieties at O-2 of the xylose residues (C. Ohsumi and T. Hayashi (1994) Plant and Cell Physiology 35:963-967; G. J. McDougall and S. C. Fry (1994) Journal of Plant Physiology 143:591-595; J. L. Acebes et al. (1993) Phytochemistry 33:1343-1345). On acid hydrolysis, the xyloglucan extracted from cotton fibers yielded glucose, xylose, galactose and fucose in the ratio of 50:29:12:7 (Hayashi et al., 1988). Xyloglucanase activity is not included in the classification of enzymes provided by the Enzyme Nomenclature (1992). Hitherto, this enzymatic activity has simply been classified as glucanase activity and has often been believed to be identical to cellulolytic activity (EC 3.2.1.4), i.e. activity against .beta.-1,4-glycosidic linkages in cellulose or cellulose derivative substrates, or at least to be a side activity in enzymes having cellulolytic activity. However, a true xyloglucanase is a true xyloglucan specific enzyme capable of catalyzing the solubilisation of xyloglucan to xyloglucan oligosaccharides but which does not exhibit substantial cellulolytic activity, e.g. activity against the conventionally used cellulose-like substrates CMC (carboxymethylcellulose), HE cellulose and Avicel (microcrystalline cellulose). A xyloglucanase cleaves the beta-1,4-glycosidic linkages in the backbone of xyloglucan