Reduction of Cu (II) in protein assays

[ragnar.flengsrud at ibf.nlh.no]

 Hi  protein bionetters !,

Could anyone out there answer this ?:

The protein assays biuret, Lowry and BCA all involves the reduction of Cu 2+ ti
Cu + in alkaline environment, but what is oxidized in the protein ? 
Where does the electron come from ?
In the article where the BCA-method is introduced (ref.: P.K. Smith et al.,
Anal. Biochem. (1985), 150, 76-85), the following statement is made (p.78): 
" While the mechanism of the Lowry reaction is not well understood, we have
theorized that it involves a reduction of the Cu 2+ to Cu 1+ at the
complexation sites within the protein molecule. "

Is this as far as the knowledge goes today as well ?

Regards,

Ragnar Flengsrud,
Agricultural University of Norway, Department of Biotechnological Sciences,
Section of Chemistry,
P.O. box 5040, N-1432 Aas, Norway
biorf at nlh10.nlh.no