How to find the concentration of an enzyme?
The enzyme unit U is not a measure of enzyme concentration but its activity, defined as the conversion of 1 micro mole ($\mu$mol) of substrate per minute (under a specified condition such as pH and T)

Absorbance (O.D. at a specific wavelength) of the enzyme is a measure of enzyme concentration, regardless of its activity. Depending on the unit of the extinction coefficient, Absorbance can be converted directly by Beer's Law to enzyme concentration, typically in mg/mL or in the standard mM.

5U/mg is the specific activity of pectinase, and the bottle contains a total of 5 kilo U (KU), meaning that there is a equivalent of 1g of active enzyme inside (often in powder). If you assume all pectinase molecules inside the bottle are active, when dissolved in 10mL water, you have a pectinase concentration of 100mg/mL. If the molecular weight of pectinase is 31kDa (31000 g/mol), then 100mg/mL is the same as 3.2mM in concentration.

By the way, Lowry protein assay measures enzyme concentration, not activity in U while DNS assay measures activity (of enzyme that produces reducing sugars), not enzyme concentration.