METHOD OF PREFERENTIAL LABELLING OF A PHYCOBILIPROTEIN WITH A SECOND DYE TO ENABLE MULTIPLE COLOR ASSAY

Abstract: A method for preparing a phycobiliprotein-Texas Red conjugate which overcomes the energy transfer/fluorescent quenching dilemma is disclosed. A phycobiliprotein, such as phycoerythrin, is conjugated with dye, such as Texas Red, in the presence of a selective salt which causes a hydrophobic intramolecular rearrangement of the phycobiliprotein thereby exposing more hydrophobic sites for binding to Texas Red. The conjugate is useful in multiple color fluorescence assays without requiring the use of multiple exciting sources.

Description: MEIHOD OF PREFERENTIAL LABELLING OF A PHYCOBILIPROTEIN WITH A SECOND DYE TO ENABLE MULTIPLE COLOR ASSAY TECHNICAL FIELD This invention relates generally to electron donor- acceptor conjugates suitable for use in multiple color assay methods, and particularly to a method of producing phycobiliprotein-dye conjugates suitable for such use. BACKGROUND ART The technique of fluorescence was first introduced by Coons in 1941. He used a blue fluorescing anthracene compound coupled to pneumococcus antiserum to detect bac¬ terial antigens in tissue section. Subsequent to this initial discovery, many fluorescing materials have been investigated, but only two, the fluorochromes fluorescein and rhodamine, are widely used, particularly in the form of fluorescein isothiocyanate (FITC) and tetramethylrho- damine isothiocyanate (TRITC) respectively. FITC covalently binds to proteins at alkaline pH through the epsilon amino residues of lysine and through terminal amino groups. It adsorption maximum is at 490-495 nm and it emits its characteristic green color at 517nm. TRITC likewise binds to proteins, has its absorption maximum at 541 nm and emits its characteristic red color at 572 nm. Fluorescence is the emission of light of one wavelength (color) by a substance that is being ir¬ radiated by light of a different wavelength. The emitted light is always of lower energy, hence longer wavelength, then the incident light. In clinical use, the strength of the fluorescence is dependent on the efficiency with which the fluorochrome transforms incident light into emitted light, the amount of dye present in the specimen under observation and the intensity of the incident light. The dye known as Texas Red has previously been investigated for clinical use in conjugation with phycoerythrins, but major problems were encountered. These problems were low fluorescent efficiency, inade¬ quate energy transfer from the phycoerythrin to Texas Red and the instability of the phycoerythrin-Texas Red con- jugate. Phycoerythrin-Texas Red conjugates are desirable, however, because the overlap of their ab¬ sorption and emission spectra have the potential to give a strong fluorescence signal. Low fluorescent efficiency occurs whenever fluores- cent chromophores are spatially adjacent to each other. It is usually called concentration quenching. See R. P. Hughland, "Excited States of Biopolymers", R. F. Steins, Ed., p 47 (Plenum Press, New York, 1983). However, high levels of labelling, resulting in chromophores being spatially adjacent to each other, are required in order to assure adequate energy (electron) transfer from the phycoerythrin to the acceptor dye chromophore. The net result is that the trade off required by the opposing ef¬ fects results in less than optimal performance. Recently, A. N. Glazer et al. have covalently linked a phycoerythrin to an allophycocyanin to produce a highly fluorescent tandem conjugate with an energy transfer ef¬ ficiency of 90%. See A. N. Glazer et al., T.I.B.S, 9:423 (1984); Biophysics J., 43,386-386 (1983); and U.S. Patent No. 4,542,104 (See also U.S. Patent No. 4,520,110 to L. Stryer et al. describing the use of phycobiliproteins as fluorescent probes for the analysis and separation of molecules and cells). However, forming a conjugate from two naturally occurring pigments derived from algae is much different from conjugating a synthetic dye such as Texas Red. In fact, the procedures usually followed for conjugating reactive dyes to proteins does not work with phycoerythrin-Texas Red. Using such procedures, one ob¬ tains a complex with a low energy transfer efficiency at low levels of labelling or fluorescence quenching at high levels of labelling. Phycoerythrin-Texas Red conjugates are known and are commercially available. For example, the phycoerythrin- TM Texas Red conjugate known as DuoCHROME is available bound to streptavidin from Becton Dickinson Immunology Systems, Mountain View, California (Catalog No. 9026). The available conjugates, however, suffer from the fact that they do not have uniform phycoerythrin-Texas Red ratios throughout the individual conjugate members. There are present overlabelled and underlabelled species as well as species having the desired or optimum degree of labelling. Consequently, energy transfer/quenching problems can arise depending upon the distribution of labelled species within the entire sample. This invention solves the energy transfer/quenching problem encountered in phycoerythrin-Texas Red conjugates by preferentially labelling sites close to the chromop- hore regions of a phycoerythrin with Texas Red and separating overlabelled and underlabelled conjugates from conjugates having the desired degree of labelling by exploiting the differences in hydrophobic character of conjugates having different degrees of labelling. DISCLOSURE OF THE INVENTION A method is provided for preparing a phycobiliprotein-reactive dye conjugate which overcomes the problems relating to the energy transfer/fluorescent quenching phenomenon encountered in such conjugates. A reactive dye, such as Texas Red, is reacted with a phycobiliprotein, such as phycoerythrin, in the presence of a salt especially selected to cause an intramolecular rearrangement of the phycobiliprotein structure whereby to expose a multiplicity of sites in its hydrophobic region with which said dye can bind to form the desired conjugate. The reaction is controlled as to the anion of the selected salt, permitted time of reaction and tem¬ perature. Conjugates having the preferred degree of phycoeythrin-Texas Red conjugation are separated from overlabelled and underlabelled conjugates by hydrophobic interaction chromatography. BEST MODE FOR CARRYING OUT THE INVENTION The first feature of this invention, preferential site labelling, makes it possible to obtain a satisfac¬ tory level of electron transfer from a phycoerythrin to Texas Red even at low levels of Texas Red conjugation by bringing the dye and the chromophore of the phycoerythrin into close proximity. This is accomplished by making use of the hydrophobic tetrapyrrole (bilin) chromophores that biliproteins are known to possess. See R. McColl and D. Guard-Frier, Phycobiliproteins, Chapter 1, C.R.C. Press (1987). Specifically, when certain anions commonly used in some "salting-out" processes are added to a phycobiliprotein containing buffer solution, they cause the phycobiliprotein to undergo an intramolecular struc- tural rearrangement which "open-up" or "exposes" hydro¬ phobic sites on the protein by reducing steric hindrance about the site. As a result of this hydrophobic intramolecular rearrangement, the sites close to chromo¬ phores can more readily react with a reactive dye, such as Texas Red, to form a conjugate. The common ions used in this process may be any of the common ions used in "salting-out" processes, such as phosphate, acetate, citrate, sulfate, tartrate and the like. The preferred anions are sulfate, phosphate and acetate. The most preferred anion is sulfate because it has little or no effect on the pH of the solution. Typically, when using one of the preferred anions such as sulfate in a phycoerythrin-Texas Red (PETR) conjugation reaction, it was found that an anion concentration in the range of about 1% to about 4% in the reaction solution resulted in a PETR conjugate having significantly improved energy transfer efficiency as compared to a PETR control con¬ jugate prepared in the absence of a preferred anion. The phycobiliprotein and the reactive dye are reacted together for a time in the range of 10 minutes at a pH greater than 7 and at a temperature of about 4°C prior to sampling to determine if an overall adequate phycobiliprotein-dye conjugation ratio has been reached. The determination is carried out by chromatographically desalting a sample of the reaction mixture and spectros- copically determining the A /A value. If the value 565 595 is in the range of 2.9 to 3.2, the reaction mixture is quenched and chromatographcally desalted. The separation of overlabelled and underlabelled conjugate species from those having the desired degree of labelling was accomplished using hydrophobic interaction chromatography with an appropriate column medium like butyl toyopearl. The PETR conjugate produced by this method can be used in conjunction with an antibody to stain different types of cell. The cells so stained will be dependent upon the choice of antibody. The importance lies in the fact that the PETR conjugates provides for a third color in fluorescence analysis with the use of only a single excitation wavelength of 488 nm. Thus the ex¬ pense of multiple excitation sources is eliminated. PREFERRED EXAMPLE In a typical, purified R-Phycoerythrin {66.6 mg/ml in PBS (Phosphate Buffered Saline) containing 2 mM EDTA} (3.0 g, 45.04 ml) was cooled in ice-bath and treated dropwise, with stirring, with an ice-cold solution of PBS containing 2 mM EDTA (29.25, ml) 20% Na SO (pH 7.0, 6.0 ml) and 1 M Potassium Borate (pH 9.80, 30 ml). To the resulting mixture was added with vigorous stirring and at 4°C a 25-fold molar excess of Texas red (20mg/ml in an¬ hydrous Dimethyl Formamide). The reaction was monitored by drawing 10 l samples periodically and desalting them on a 0.5 - 2 ml Sephedex G-50 column in PBS. The protein containing peak was collected and its A565/A595 value determined spectrophotometerically. If the A565/A595 values remain above 3.2, even after 30 mins or more of reaction time, a further aliquot of Texas red solution was added to the reaction mixture. When A565/A595 value fell below 3.2, preferably in the range of 2.9-3.2, reac¬ tion was quenched by addition of an one-hundred fold molar excess of glycine to the reaction mixture. The reaction mixture was next desalted on a Sephadex G-50 column in PBS, 2mM EDTA. The phycoerythrin-Texas red conjugate, in the protein peak, was then chromatographi- cally factionated on a butyl 650 M chromatographic column by eluting with reverse gradient (3% to 0%) of Sodium Sulfate in 100 mM potassium phosphate solution containing 2 mM EDTA at pH 7.0 ± 0.1. Chromatographic fractions having the desired emission characteristics (high energy transfer and high quantum efficiency) were pooled, con¬ centrated, desalted and reconcentrated to give a purified phcoerythrin-Texas Red conjugate. The purified PETR con¬ jugate was used as a marker in fluorescent immunoassays. The PETR marker conjugates to protein-like substance such as antibodies and streptavidin using methods known in the art.

Claims: CLAIMS 1. A method for producing a phycobiliprotein- reactive dye conjugate comprising, reacting said dye with a phycobiliprotein in the presence of at least a selec¬ tive salt which effects an intramolecular rearrangement of said phycobiliprotein to expose multiple dye reactive sites, desalting the reaction and separating the phycobiliprotein-dye conjugate by a chromatographic means. 2. The method in accordance with claim 1 wherein the phycobiliprotein is phycoerythrin. 3. The method in accordance with claim 1 wherein the dye is Texas Red. 4. The method in accordance with claim 1 wherein the salt is one whose anion is selected from the group consisting of phosphate, sulfate, acetate, citrate and tartrate ions. 5. A method for producing a phycobiliprotein-dye conjugate suitable for use in a multiple color assay comprising reacting a phycobiliprotein containing solution of pH greater than 7 and an excess of a dye in the presence of about 1% to about 4% of a salt at a tem¬ perature of about 4°C, desalting said reaction and separating the phycoerythrin-dye conjugate by a chromatographic means. 6. The method in accordance with claim 5 wherein said phycobiliprotein is phycoerythrin. 7. The method in accordance with claim 5 wherein said dye is Texas Red. 8. The method in accordance with claim 5 wherein said salt is a salt whose anion is selected from the group consisting of phosphate, acetate, sulfate, citrate, and tartrate. 9. A method for producing a phycobiliprotein-dye conjugate comprising reacting an excess of dye with the phycobiliprotein at a pH of greater than 7, desalting and separating various populations of phycobiliprotein-dye conjugates by hydrophobic interaction chromatography. 10. The method in accordance with claim 9 wherein the phycobiliprotein is phycoerythrin. 11. The method in accordance with claim 9 wherein the dye is Texas Red. 12. A phycobiliprotein-reactive dye conjugate prepared by the process comprising mixing phycobili¬ protein solution and a reactive dye solution in the presence of a selected salt, which salt effects an intramolecular rearrangement of the phycobiliprotein to expose multiple dye reactive sites; spectrophotometric- ally monitoring the A565/A595 band ratio of the reaction mixture; allowing the reaction mixture to react for a sufficient time to achieve an A565/A595 band ratio in the range of 2.9-3.2; quenching the reaction mixture; desalt¬ ing the quenched reaction mixture; and isolating the phycobiliprotein dye conjugate by chromatographic separation. 13. The phycobiliprotein-reactive dye conjugate in accordance with claim 12 wherein the phycobiliprotein is phycoerythrin. 14. The phycobiliprotein-reactive dye conjugate in accordance with claim 12 wherein the reactive dye is Texas Red. [received by the International Bureau on 26 March 1992 (26.03.92); original claims 10,11,13 and 14 cancelled; original claims 1,5,9,12 amended; new claims 11 and 12 added; other claims unchanged (2 pages)] 1. A method of producing a phycobiliprotein-amine reactive dye conjugate comprising, reacting, in a buffer solution of pH greater than 8, an excess of said dye with a phycobiliprotein in the presence of a selected salt which effects an intramolecular rearrangement of said phycobiliprotein to expose multiple sites reactive with said dye, desalting the reaction and removing unreacted excess dye, and separating the various populations of the phycobiliprotein-amine reactive dye conjugate by a chromatographic means. 2. The method in accordance with claim 1 wherein the phycobiliprotein is phycoerythrin. 3. The method in accordance with claim 1 wherein the dye is Texas Red. 4. The method in accordance with claim 1 wherein the salt is one whose anion is selected from the group consisting of phosphate, sulfate, acetate, citrate and tartrate ions. 5. A method of producing a phycobiliprotein-amine reactive dye conjugate suitable for use in a multiple color assay, the method comprising reacting, in a buffer solution of pH greater than 8, a phycobiliprotein and an excess of an amine-reactive dye in the presence of at least one selected salt which effects an intramolecular rearrangement of said phycobiliprotein to expose multiple sites reactive with said dye, desalting the reaction and removing excess unreacted dye, and separating the various populations of phycobiliprotein-amine reactive dye con¬ jugates by hydrophobic interaction chromatography. 6. The method in accordance with claim 5 wherein said phycobiliprotein is phycoerythrin. 7. The method in accordance with claim 5 wherein said dye is Texas Red. 8. The method in accordance with claim 5 wherein said salt is a salt whose anion is selected from the group consisting of phosphate, acetate, sulfate, citrate, and tartrate. 9. A method producing a phycoerythrin-Texas Red conjugate comprising reacting an excess of Texas Red with the phycoerythrin at a pH of greater than 8 in the presence of about 1% to about 4% of at least one selected salt, monitoring the reaction between Texas Red and phycoerythrin by means of the reaction's adsorption spectrum at wavelengths of 565 and 595 nanometers, quenching the reaction mixture at such time as monitoring indicates that a ratio of A /A reaches a value in 565 595 the range of 2.9-3.5, removing excess dye and separating the population of phycoerythrin-Texas Red conjugate by hydrophobic interaction chromatography. 10. A phycobiliprotein-reactive dye phycoerythrin- Texas Red conjugate prepared by the method of claim 9. 11. The conjugate prepared by the method of claim 1. 12. The conjugate prepared by the method of claim 5.

Inventors: GUPTA RAVINDER K, GUPTA, RAVINDER, K.
Applicants: COULTER CORP, COULTER CORPORATION
Application year: 1991
Main classification: G01N33
IPC codes: C07K1, C07K14, C07K17, G01N33