HUGO THEORELL 



tyrosine may be important in the quenching of FMN. O.Y.E. 

 does not contain any reactive sulfhydryl groups. Therefore, if 

 iodine were added to the apoprotein it could be expected to 

 enter mainly into the 3- and 5-positions in the benzene ring of 

 tyrosine, thereby greatly increasing the acidity of the tyrosine 

 hydroxyl group. In fact it was found that very small quantities 

 of iodine were sufficient to decrease the coupling velocity of 

 FMN to apoprotein very substantially. Ninety per cent of the 

 disappeared amount of iodine was recovered in diiodotyrosine. 

 The results suggest that in the O.Y.E. tyrosine-OH may be 

 attached to the imino group (13) — or its enol form — presumably 

 by a hydrogen bond. This does not exclude the possibility that 

 other groups as well are important in this binding. 



There is at present no method available for studying the 

 kinetics of the reaction between leuco-FMN (FMN-H2) and the 

 apoprotein. However, the remarkably high normal potential 

 of the O.Y.E., as determined by Kuhn and Boulanger (6), 

 — 0.06 v., as compared with that of free riboflavin, —0.185 v., 

 indicated that the leuco-O.Y.E. should be around 10,000 times 

 less dissociated than the O.Y.E. In the presence of suitable 

 anions, however, measurable values of the dissociation constant 

 could be expected, and some information on the influence of 

 salts on kjkx for leuco-O.Y.E. could be hoped for. G. S. Vest- 

 ling, while working in this laboratory in 1954, therefore under- 

 took a reinvestigation of the normal potential of O.Y.E. Vest- 

 ling (33) found E'q at j&H 7 and 30° C. of -0.1 23 ±0.006 v., 

 thus 63 mv. lower than Kuhn's and Boulanger's value. The 

 slope, E'j^pU was found = -0.06 v. from/)H 4.8 to 8.9 



The new value sets the ratio of the dissociation constants 

 of O.Y.E. and leuco-O.Y.E. a little higher than 100. 



As could be expected, the anion effects were soon found to 

 be of more general occurrence. Walaas and Walaas (34), in 

 this institute, found strong inhibitory effects of anions on the 

 activity of af-amino acid oxidase in the following order : 



I- > Br- > NO3- > CI- > P04= > SO4- > GH3COO- » F- 



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