Question

what does a high value for miller units mean

Answer 1

In 1972, Jeffrey Miller published "Experiments in Molecular Genetics" which contained a protocol for determining the amount of β-Gal with o-nitrophenyl-β-D-galactoside. Because of this, ONPG/β-Gal assays are referred to as "Miller" assays, and a standardized amount of β-Gal activity is a "Miller Unit". Thus one miller unit is defined as-

1 Miller unit = 1000 x (Abs420 - ( 1.75 x Abs 550)) / t x v x Abs 600

where

Abs 420 is the absorbance of the yellow o-nitrophenol,

Abs 550 is the  is the scatter from cell debris, which, when multiplied by 1.75 approximates the scatter observed at 420nm,

Abs 600 shows cell density

t - reaction time in minutes

v- the volume in ml.

Beta-galactosidase acts on the colourless substrate o-nitrophenyl-β-D-galactoside to form yellow chromophore o-nitrophenol(ONP) as the hydrolytic product Thus the amount of ONP produced as a function of reaction time per volume of cell culture is divided by the optical density of the culture to generate a value of specific enzyme activity in Miller units.

high value for miller units means that the enzyme is highly active and it converts its substrate into product fast per unit time i.e it has a high rate of reaction.