ANALYTICAL SCIENCES AUGUST 1989, VOL. 5 371 Reviews High Performance Liquid Chromatography of Bioactive Substances Using Bifunctional Fluorogenic Reagents for Derivatization A Review Yosuke OHKURA Faculty of Pharmaceutical Sciences, Kyushu University62, Maidashi, Fukuoka 812, Japan Pre- and postco umn derivatization methods employing recently-developed Bifunctional fluorogenic reagents for high performance liquid chromatography have been demonstrated to be successfulregarding both sensitivityand selectivity in the determination of bioactive substances and their related enzymes. The reagents were designed mostly based on the principle that vicinal or geminal, homo- or hetero-Bifunctional (diamino, amino-imino, amino- sulfhydrylor amino-hydroxyl) compounds react with 1,2-dioxo compounds (1,2-ketols, glyoxals, 1,2-diketones or 1,2-quinones) or monooxo compounds (aldehydes) to yield fluorescent cyclic products. These reagents have been introduced into the quantification process of diverse bioactive substances in biological samples at the femtomole level. Keywords Bifunctional f uorogenic reagent, bioactive substance, enzyme activity, serum, urine, animal tissue, high performance liquid chromatography, quantification, high sensitivity, femtomole level, high selectivity, simplicity, precolumn derivatization, postcolumn derivatization analysis methods should be both selective and sensi- 1 Introduction tive. Selectivity and sensitivity have been achieved by means of traditional analytical separation methods, A great variety of bioactive substances that exhibit including: gas chromatography (often coupled with bioactivities, even at extremely low concentrations, mass spectrometry), liquid chromatography (coupled occur in biological fluids and tissues, and their with mass spectrometric, refractive index, UV, fluores- concentrations are generally controlled by their related cence and electrochemical detection and various other enzymes. Therefore, it is of importance in areas where detection systems) and elctrophoresis. Also, molecular life science is involved to measure these substances and recognition techniques based on bioaffinity, such as their biosynthetic and metabolizing enzyme activities in immunoassay and receptor assay, have been used. biological materials. Since such biosubstances and Derivatization techniques used to lead target analyte(s) enzymes are usually found in complex matrices, to isotopically labeled, UV-absorbing, fluorescent, 372 ANALYTICAL SCIENCES AUGUST 1989, VOL. 5 chemiluminescent or electroactive compound(s) are group (diamino, amino-imino, amino-sulfhydryl or very useful for sensitive and selective detection when amino-hydroxyl compounds) may combine with mono- combined with the various separation methods. oxo compounds (aldehydes) or 1,2-dioxo compounds High performance liquid chromatography (HPLC), (1,2-ketols, glyoxals, 1,2-diketones or 1,2-quinones) to one of the most operable separation methods, has been afford fluorescent cyclic derivatives (i.e. imidazoles, widely accepted as a potential analytical tool for thiazoles, oxazoles, quinoxalines orp-oxazines) (Fig. 1). routine use. Regarding its detection systems, fluores- These bifunctional compounds and 1,2-dioxo com- cence detection is not only highly sensitive but also pounds can be reagents and analytes, respectively, and selective; it has therefore been introduced in the deter- vice versa. In addition, on the basis of the chemical mination process of bioactive substances in biologi- structures of fluorophores derived from reactions cal samples. A non-fluorescent or weakly fluorescent between bifunctional reagents and 1,2-dioxo com- analyte must be converted into highly fluorescent pounds, some monofunctional reagents have been derivative(s), normally through its functional group(s) contrived. by reaction with a fluorogenic reagent. This article aims to review sensitive HPLC analyses Fluorescence derivatization HPLC can be categoriz- of bioactive substances of small molecules, including ed into precolumn (before separation) and post- therapeutic drugs, in biological materials by means of column (after separation) derivatization. A precolumn pre- or postcolumn fluorescence derivatization using derivatization reaction does not require a rapid termina- these reagents as well as some reagents reported so far. tion and its conditions can usually be optimized. A postcolumn derivatization reaction, on the other hand, should proceed rapidly, but not necessarily terminate, 2 Aromatic Aldehyde Precursors and Biogenic since a prolonged reaction time causes peak-broaden- Amine-Related Enzymes ing, thus yielding low resolution. Fluorogenic reagents for postcolumn derivatization need to be non-fluores- 2.1 Aromatic aldehydes and arylaliphatic aldehydes cent or markedly different from the derivatives in derivable from drugs fluorescence excitation and/or emission spectra in the 1,2-Diaminonaphthalene, an aromatic 1,2-diamino mobile phase; they are allowed to give multiple compound, was first found to react with aromatic fluorescent derivatives, provided that their reactions are aldehydes in an acidic medium9 to form 2-substituted reproducible. naphtho[l,2-d] imidazole derivatives.10 Since then, o- A number of fluorogenic reagents which meet the phenylenediamine11,1,2-diamino-4,5-dimethoxy benzene12 above-mentioned requirements have already been de- (DDB, Fig. 2),1,2-diamino-4,5-methylenedioxybenzene13 veloped, and are summarized in reviewsl-6 and books.7'8 (DMB, Fig. 2) and 1,2-diamino-4,5-ethylenedioxyben- Many of these reagents have been applied to pre- zene13 (DEB, Fig. 2) have been developed (in that and/ or postcolumn fluorescence derivatization in the order) as fluorogenic reagents for aromatic aldehydes, HPLC of biological compounds. We have also which all produce the corresponding fluorescent introduced several fluorogenic reagents based on the imidazole compounds in acidic media (Fig. 2). following principle: compounds having vicinal or DDB also reacts with such arylaliphatic aldehydes as geminal bifunctional groups, one of which is the amino phenylacetaldehyde and cinnamaldehyde and has, thus, Fig. 1 Scheme of reactions between bifunctional-bifunctional, or bifunctional-monofunc- tional compounds. Ar., aromatic moiety; Aliph., aliphatic moiety. ANALYTICAL SCIENCES AUGUST 1989, VOL. 5 373 Fig.2 Aromatic 1,2-diamines and 1,2-aminothiols and their reactions with aromatic aldehydes. useful for evaluating the placental and foetal functions. DEB is the most sensitive fluorogenic reagent for aromatic aldehydes, especially those having a phenolic group. Forphenicine [L-(4-formyl-3-hydroxyphenyl)- glycine], an inhibitor of alkaline phosphatase, has a formylphenolic group in the molecule and is readily derivatized with DEB into a fluorescent imidazole compound which is separable on a reversed-phase Fig. 3 Fluorescence derivatization of phenolic compounds (ODS) column. Thus, an HPLC method has been involving the Reimer-Tiemann reaction followed by the DDB established for monitoring forphenicine in mouse serum reaction. and muscle.18 2'2 Amine-metabolizing enzyme activities 2,2'-Dithiobis(1-aminonaphthalene)19 (DTAN) reacts been applied to a precolumn derivatization reversed- with high sensitivity with aromatic aldehydes in an phase (ODS column) HPLC determination of bestatin acidic medium in the presence of tributylphosphine, [(2S,3 R)-3-amino-2-hydroxy-4-phenylbutanoyl-S-leu- which serves to reduce the reagent to 1-amino-2- cine], an aminopeptidase inhibitor, in human14 and thionaphthol, to produce 2-substituted naphtho[1,2- mouse15 sera and mouse muscle15, which can be derived d]thiazole compounds20 (Fig. 2). The reaction is to phenylacetaldehyde by periodate oxidation. similar to that of 2-aminothiophenol2l (Fig. 2), but p-Hydroxybestatin, an active metabolite of bestatin, occurs under much milder conditions (above 0° C) and has a phenolic group in the molecule; the compound in can afford more intense fluorescence. human16 and mouse15 sera and mouse muscle15 can be DTAN have been applied to the highly sensitive monitored by reversed-phase (ODS column) HPLC assay of dopamine-/3-hydroxylase (DBH) activity in rat with precolumn fluorescence derivatization with DDB serum and adrenal medulla22 and human serum:23 after converting the phenolic group into an o-formyl- octopamine enzymatically formed from the substrate phenolic group by the Reimer-Tiemann reaction (see tyramine is oxidized with periodate to p-hydroxybenzal- Fig. 3 for the reaction scheme). Oestrogens [oestriol dehyde, which is determined by normal-phase HPLC (oesta-1,3,5(10)-trien-3,16a,16f 3-triol), oestrone (3-hy- on an alumina column after conversion into the DTAN droxyoesta-1,3,5(10)-trien-l7-one) and oestradiol (oesta- derivative. 1,3,5(10)-trien-3,17 f3-diol)] have a phenolic group in Monoamine oxidases A and B in rat brain mito- their molecules; the free and total (the sum of free and chondria can be assayed:24 p-sulfamoylbenzaldehyde conjugated) oestrogens in pregnancy urine can be formed enzymatically from p-sulfamoylbenzylamine (a simultaneously measured based on the same principle newly developed substrate for monoamine oxidase A25) as shown in Fig. 3.17 The concentration values are and benzaldehyde produced enzymatically from benzyl- 374 ANALYTICAL SCIENCES AUGUST 1989, VOL. 5 diamine Fig. 5 Reactions of o- phenylenediamine, DDB and DMB with a-keto acids. Fig. 4 DNT and its reaction catalyzed by COMT. amine (a known substrate for monoamine oxidase B) are simultaneously converted into fluorescent