# What does units/mg mean for Streptavidin

I got streptavidin for surface reaction. The label says "biotin binding: 16 units/mg". What does units/mg mean?

Does it mean "1 mg biotin can bind to 16 units SA"?

How much is the unit here?

• From which company do you have this product?
– Chris
Jan 28, 2015 at 10:18
• @Chris from invitrogen Jan 28, 2015 at 11:05

16 units/mg means 16 units per milligram of protein.

Many companies, including Invitrogen, define 1 unit streptavidin as the amount of streptavidin necessary to bind 1 microgram of biotin.

• Thank you for your answer. Then 16 units/mg actually means 1 gram of SA could bind to 16 gram of biotin? How come this happen? The molecular weight of SA is hundred times larger than of biotin, is it possible to have thousands of binding position? Jan 28, 2015 at 11:15
• "Milli" means one-thousandth, "micro" one-millionth :)
– AliceD
Jan 28, 2015 at 11:18
• @leetom The units are not same. The label says 16 units/mg. You must find out how much 1 unit of biotin weigh to make a comparison. For example if one unit of biotin weighs 1/16 milligram then 1 mg SA will bind to 1 mg biotin. Jan 28, 2015 at 14:59
• @CRags ChrisStronks, Ah sorry, I made the mistake ;) Jan 29, 2015 at 8:20

1 U SAV can bind 1 ug biotin

This tells you that in a 16 U/mg SAV sample, every mg of SAV will bind 16 ug of biotin. You can figure out the molar ratio from this:

$16\mu g\ BIO\cdot\frac{1mol\ BIO}{244310000ug\ BIO}\cdot\frac{52800000mg\ SAV}{mol\ SAV}$

Which equals:

$\frac{3.46mol\ BIO}{mol\ SAV}$

Theoretically, each SAV tetramer can bind 4 biotins. This tells you that there is some heterogeneity in the sample and that, on average, each SAV tetramer can bind 3.46 biotins.

• it does not reflect protein heterogeneity, but sample impurity (some 14%). Probably mostly water or salts. +1 for the clear calculation.
– AliceD
Jan 28, 2015 at 22:42
• @ChrisStronks Yes it may be true that some amount of the sample is be salt, depending on if a buffer was used prior to lyophilisation. However, reduced binding capacity may certainly be due to protein heterogeneity. One, biotin is essential to all organisms and, during production of SAV, it is expected that some amount will bind to endogenous biotin, using up a binding pocket. This cannot be accounted for in the common binding assays. Two, depending on the method, purification of SAV can require harsh elution conditions which can deform some of the binding pockets. Jan 29, 2015 at 4:03
• that's interesting. Thanks for the explanation.
– AliceD
Jan 29, 2015 at 7:48
• @canadianer Thank you for the explanation! :) Jan 29, 2015 at 8:29