= Dye-linked enzyme assay
A \b[dye-linked enzyme assay] couples an <enzyme> reaction to a measurable change in a colored reporter. A dye can accept or donate reducing equivalents directly or through an intermediate <electron carrier>. The optical signal belongs to the coupled system, so its interpretation requires both the enzyme chemistry and the reporter chemistry.
The original <methanol dehydrogenase> study used <phenazine methosulfate> and <DCPIP> in a coupled spectrophotometric measurement. It separately investigated product identity because an electron-transfer readout alone did not establish which chemical species accumulated.<The microbial oxidation of methanol. 2. The methanol-oxidizing enzyme of Pseudomonas sp. M 27 by C. Anthony and L. J. Zatman>[Materials and Methods, spectrophotometric assay; p.620, product analysis]{ref}
The <Beer-Lambert law> provides a quantitative model. Let $\varepsilon$ be the dye's absorption coefficient, $\ell$ the <optical path length>, and $\nu$ the number of reporter molecules changed per substrate molecule consumed. With an applicable blank-corrected absorbance slope,
$$
v_{\mathrm{substrate}}=\frac{|dA/dt|}{\varepsilon\ell\nu}.
$$
For a stipulated coefficient $20\,\mathrm{mM^{-1}cm^{-1}}$, path $0.50\,\mathrm{cm}$, slope $0.10\,\mathrm{min^{-1}}$ and $\nu=2$, the inferred substrate-concentration rate is $0.005\,\mathrm{mM\,min^{-1}}$, or $5\,\mathrm{\mu M\,min^{-1}}$. These are model values, not the cited experiment's reagent constants.
The conversion assumes that the reporter response is linear, its stoichiometry is known and the coupling reaction keeps up with the measured reaction. A slow coupling step can become the bottleneck; direct dye reduction or sample color can add background. <Blank correction>, substrate omission and enzyme omission test different contributions rather than serving as interchangeable controls.
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