Histamine is found in many biological systems and plays a major role in the generation of symptoms of allergic and inflammatory disease. It is known that histamine is involved in the immune response, in secretion of gastric acid, and in the functioning of the nervous system. Histamine is released from granules contained within tissue mast cells and circulating basophils. The release of histamine is often a reaction to the association of an antibody on the surface of such a cell with an allergen. Released histamine interacts with many body tissues to induce changes which are symptomatic of several diseases. It is well established that the amount of histamine released is an excellent indicator of the presence and severity of certain diseases. For example, the amount of histamine release provoked by an allergen is an excellent indicator of the degree of allergic sensitivity to the allergen. Measurement of the amount of histamine released in response to challenge by an allergen is not commonly used as a diagnostic method because of the inadequacies of currently available methods of histamine quantification.
There is a wide range of methods available in the prior art for the quantitative measurement of histamine. However, none of the available methods are adequate for routine use in a clinical laboratory. The most widely used assays are variations of an ortho-phthalaldehyde conjugation assay. In this method histamine in a body fluid such as blood or urine is separated from other biological amines by a series of organic-inorganic phase extractions with concomitant shifts in pH. The isolated histamine is then reacted with ortho-phthalaldehyde to provide a fluorescent product which is quantified. This procedure is labor-intensive, and when automated, requires major capital investment and dedication of capital equipment.
An alternative histamine assay is an enzymatic isotopic assay wherein a radioisotope tag is attached to histamine by radioisotope-labeled adenosylmethionine in the presence of methyltransferase. This assay also requires an extraction procedure. It is time consuming, labor-intensive, and presents the usual problems associated with the use of radioisotopes.
Less commonly used histamine assay methods include high pressure liquid chromatography of fluoresceamine-conjugated histamine, various gas chromatographic assays, and a thin layer chromatographic assay. While these methods are more or less useful in research laboratories, none combines the features necessary for a clinical laboratory assay method, such as accuracy, speed, ease of operation, low cost, and high volume.
Receptor-based assays are known in the art. U.S. Pat. No. 3,817,837 relates to an assay method for determining the presence of a ligand which may be histamine. The ligand is bound to an enzyme and the enzyme-bound ligand is required to undergo a substantial reduction in enzymatic activity when binding to a receptor. In contrast, the preferred histamine-indicator conjugate of the present invention maintains its activity or even undergoes stimulation of activity when bound to a receptor.
The only commercially available receptor-based assay is believed to be Biocept-G.RTM. (Wampole Laboratories, Dist. Div. of Carter-Wallace, Inc., Cranbury, N.J.). Biocept-G.RTM. is a pregnancy test based on the specific binding of human chorionic gonadotropin (HCG) in blood serum to preparations of plasma membranes from animal cells such as those of rat testes or bovine corpora lutea.