Exercise 5.2 - Preparation of Standard Curve
- 8 mM DOPA
- Enzyme Extract - From Exercise 5.1
- Test tubes
- 5 ml Pipette
- 0.1M Citrate Buffer, pH 4.8
- Spectrophotometer and Cuvettes
- Begin by preparing a standard solution of the orange
colored dopachrome from L-DOPA. To 10 ml of 8 mM DOPA, add
0.5 ml of your enzyme extract
and allow the solution to sit for 15 minutes at room
temperature. During this period, all of the DOPA will be
converted to dopachrome, and your solution will now contain
8 mM dopachrome. Dopachrome is somewhat unstable in the
presence of light and should be stored in an amber bottle or
out of the light.
- Prepare a 1:1 series of dilutions of the 8 mM Dopachrome to
yield the concentrations in the following table: Add 3.0 ml
of each indicated concentration to tubes #1-8.
of Dopachrome (mM)
With these dilutions, you have prepared tubes containing
concentrations from 0 to 8 mM dopachrome (tubes 1-8). Tube 1
contains no dopachrome and is used for blanking the
The units of concentration are millimolar (mM). A 1.0 mM
solution contains .001 moles per liter or .000001 moles per
ml. Thus, with a volume of 3.0 ml, there are .000003 moles
of dopachrome, or 3 micromoles. Correspondingly, tubes 2-8
contain 1 to 24 micromoles of dopachrome. For the remainder
of this exercise, be sure to distinguish between
concentration (mM) and total amount of substance present
- Turn on your spectrophotometer and set a wavelength of
475. Use tube #1 from the above dilutions as a blank and
adjust the spectrophotometer for 0 and 100% T. Read the
absorbance (or read and convert transmittance) of each of
the solutions in tubes 2-8 and complete the following table:
- Calculate the values for the last column of the table.
This column represents the simplest calculation of the
extinction coefficient for dopachrome absorbance. Average
the values in this column and enter the number at the bottom
of the column. This is the average extinction coefficient
and can be used in subsequent determinations of dopachrome
concentrations according to the Beer-Lambert law.
You can more accurately determine the extinction coefficient
by performing a linear regression analysis of your data, and
computing the slope and y intercept. The slope of the linear
regression will represent the extinction coefficient for
- Plot a scattergram of the absorbance value against the
concentration of dopachrome. The known concentration of
dopachrome should be the x axis, while absorbance should be
the y axis.
- Plot the computed slope and intercept of the linear
regression as a straight line overlaying your scattergram.
The equation for a straight line is y = mx + b, where m is
the slope and b the intercept.
Since tyrosinase catalyzes the conversion of L-DOPA to
dopachrome, this exercise measures the conversion of
colorless DOPA to the dark orange dopachrome. Substrate and
product are in a 1:1 ratio for this reaction, thus the
amount of product formed equals the amount of substrate
used. The optical density of dopachrome @475 nm is directly
proportional to the intensity of orange color formation in
solution (Beer-Lambert Law).
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Cell Biology Laboratory Manual
Dr. William H. Heidcamp, Biology Department, Gustavus Adolphus College,
St. Peter, MN 56082 -- firstname.lastname@example.org