ESTADO ACTUAL DEL CAMPO

2.10.1 Assays for chymotrypsin and trypsin

A modified version^^**^ o f the well-known Schwert and Takenaka method^^^^^ was used for the determination o f chymotrypsin and trypsin. The method is based upon the changes that occur in ultraviolet absorbance during the hydrolysis o f amino acid esters by these enzymes. N-Benzoyl-L-tyrosine ethyl ester (BTEE, 5x10^ M) was used as chymotrypsin substrate and N-a-p-tosyl-L-arginine methyl ester (TAME, 1.04x10'^ M) as trypsin substrate.

A Perkin Elmer spectrophotometer (Coleman 55) fitted with a water bath and maintained at 37 °C was used. Measurements were made using a quartz cuvette with a 1 cm path length. Substrate (3 ml) was placed into each cuvette. To the control cuvette was added water (100 pi) and to the corresponding experimental cuvettes were added chymotrypsin (100 pi, 50 pg/ml), trypsin (100 pi, 0.15 pg/ml) or any other enzyme tested. After each addition the contents o f the cuvettes were agitated for 5-10 s with a small spatula. Absorbance differences were measured at various time intervals and at the wavelengths o f 254 nm (chymotrypsin-like enzymes) or 247 nm (trypsin-like enzymes).

The most common way o f expressing enzyme activity is based on the rate o f the reaction catalysed, for example in terms o f pmol o f substrate transformed /min or pmol o f product formed/min. One unit o f any enzyme is defined as that amount which will catalyse the transformation o f 1 pmol o f substrate per minute (1 pmol/min).

2.10.2 Enzymatic assay of protease using casein as a substrate

short, casein is digested with a relevant protease under standard conditions. The undigested casein is precipitated with trichloroacetic acid (TCA) and the amount of unprecipitated split product, which is a measure o f the amount o f protease present, is determined with Folin & Ciocalteu’s phenol reagent which gives a blue colour with tyrosine.

Conditions: T = 37 ®C, pH = 7.5, A660nm, light path = 1 cm Method: Colourimetric

Reagents: A. Potassium phosphate buffer (50 mM, pH 7.5) B. Casein solution (0.65% (w/v))

C. TCA reagent (110 mM)

D. Folin & Ciocalteu’s phenol reagent (25% v/v) E. Sodium carbonate (500 mM)

F. Sodium acetate (10 mM) buffer with calcium acetate (5 mM, pH 7.5)

G. L-tyrosine standard (1.1 mM)

H. Protease enzyme solution (0.1 - 0.2 unit/ml o f cold reagent F)

Digestion

The following reagents were placed into suitable vials (ml):

Test Blank

Reagent B (casein) 5.00 5.00

Equilibrated at 37 °C, and then added:

Reagent H (enzyme solution) 1.00 --- Mixed by swirling and incubated (10 min, 37 ®C). Then added:

Reagent C 5.00 5.00

Reagent H (enzyme solution)--- --- 1.00

The tubes were mixed by swirling and incubated (30 min, 37 °C), followed by filtration through Whatman No. 50 filter paper. The solutions were later used in the colour development step.

Colour development

The standard curve was prepared by pipetting the following reagents into suitable vials (ml): S td l Std2 Std3 Std4 Std Blank Reagent G 0.05 0.10 0.20 0.40 0.00 Deionised H2O 1.95 1.90 1.80 1.60 2.00 Reagent E 5.00 5.00 5.00 5.00 5.00 Reagent D 1.00 1.00 1.00 1.00 1.00

The sample tubes were prepared for the colour development by pipetting the following reagents into suitable vials (ml):

Test Blank

Test filtrate 2.00 ---

Blank filtrate --- 2.00

Reagent E 5.00 5.00

Reagent D 1.00 1.00

The vials were vortexed and incubated (30 min, 37 ®C). After cooling (RT), the solutions were filtered through the Whatman No. 50 filter paper and the absorbance recorded immediately.

Calculations

Standard curve:

AA660 nm standard = A660 nm standard - A660 nm standard blank AA660 nm standard vs pmol o f tyrosine was then plotted.

Sample determination:

AA660 nm sample = A660 nm test - A660 nm sample blank

(^imol tyrosine equivalents released) x 11

Units/ml enzyme = ---

Ix 10 X 2

where: 1 1= total volume (ml) o f assay

10 = time o f assay (min) as per the unit definition

1 = volume o f enzyme (ml) used

2 = volume (ml) used in colourimetric determination

Units/mg solid = Units/mg protein = units/ml enzyme mg solid/ml enzyme units/ml enzyme mg protein/ml enzyme

2.11 Measurement of histamine

In experiments with RPMC, histamine was determined essentially according to the method o f Shore et al^^^^^ but with omission o f the extraction steps. In this method, the fluorescence generated by the alkaline condensation product o f histamine with o-

phthaldialdehyde (OPT) is measured. The scheme o f the reaction is given in Fig. 2.1. The fluorescence was measured with a commercially available spectrometer (Perkin Elmer LS 5B), using excitation and emission wavelengths o f 360 nm and 440 nm respectively. The fluorescence generated was proportional to the histamine content in the sample.

Briefly, NaOH (200 |il, 1 M) was added to each sample, followed by OPT (1 % w/v in methanol, 75 pi). The tubes were allowed to incubate (4 min, RT) before quenching the reaction by the addition o f HCl (3 M, 100 pi). The tubes were vortexed following each addition to ensure adequate mixing. In this manner, histamine concentrations as low as 5-10 ng/ml could be measured.

I f immunoglobulin was used as the secretagogue, particularly at high concentrations, protein precipitation often resulted on the addition o f the acid. To counteract this, samples were centrifuged (200 g, 10 min, 4 °C) before being assayed.

2.11.2 Automated assay

Tissue mast cell preparations, human basophils and RPMC samples which contained drugs which interfered with the fluorescence obtained in the manual assay, were assayed using a commercial autoanalyser (Technicon Autoanalyser II). The final assay reaction is essentially the same as the manual method described above, but the automated process includes chemical extraction steps to purify the histamine samples prior to the reaction with OPT^^^^l

Samples were prepared for analysis by acidification with perchloric acid (72 %, 50 pi) to hberate residual histamine and induce protein precipitation. The tubes were vortexed and then centrifuged (200 g, 10 min RPMC or 20 min tissue mast cells and basophils, 4 °C). The samples were then decanted into sample cups and assayed in automated sequence. Initially, the samples were made alkaline and the histamine extracted with salt saturated butan-l-ol. The organic phase was then separated and retained and washed once in a less alkaline medium. The solution was then made less polar by the addition of «-heptane and the histamine back extracted into dilute HCl. The sample was then made alkaline before being allowed to react with OPT. The fluorescent adduct was stabilised by acidification and quantified using a ftuorophotometer attached to a chart recorder. This process allowed measurement o f histamine concentrations as low