SIDXEY F. J' FUCK 119 



tion as a luiu tioii ol lenipeialure is not to be expected. 



The laclic clehydrogenase-DPNH complex. The excitation of 

 DPNH lluoiescence in the lactic dehydrogenase (LDH) comjjlex 

 is shown in Fig. 8. As with GPD, there is a strong excitation band at 

 280 myu, but in this case the excitation of the coenzyme (solid line) 

 and the excitation of the protein (open circles) show a point to 

 point coincidence after normalization at 280 ni/^. The dotted curve 

 shows the 260 n\^ excitation that would have appeared if the coenzyme 

 behaved as it does in the free form. Such a contribution could 

 easily have been detected and was not observed. By this criterion 

 the coenzyme is bound in an open conformation on LDH, as it is 

 on all other dehydrogenases that have been examined with the ex- 

 ception of GPD. The extent to which the dinucleotide is unfolded 

 in the so-called open conformation is undetermined. Unfolding and 

 disorientation of the inner complex need not be extensive in order 

 to weaken greatly the interaction responsible for the energy transfer. 



Comparisons and Interpretations 



Each pyridine-nucleotide-linked dehydrogenase has idiosyncrasies 

 of its own which may or may not be expressed in the gross charac- 

 teristics of the coenzyme complex. Some properties of a group of 

 enzymes are listed in Table 1. In so far as the general pattern is 

 concerned, most of the enzymes appear to be qualitatively similar; 

 and \\e infer that the properties selected are not ones that are closely 

 correlated Avith specificities of behavior. The aberrant case is GPD, 

 and although the amount of detailed interpretation that we can make 

 is meager it is pertinent to consider a few of the properties briefly. 



Fluorescence enhancement. This is a sign that the nucleotide in 

 the complex is shielded and that vibrational modes, and hence chances 

 for thermal deactivation, are restricted. Even in such a case, 

 however, enhancement may not occur if polarities and conformation 

 are not favorable. For example, Winer and Schwert observed that 

 oxalate enhances and oxamate quenches the fluorescence of DPNH 

 in the LDH complex (42) . These ions are inhibitory substrate ana- 

 logues and presumably interact with bound DPNH at the catalytic 

 site. Groups with the same effects may in principle be integral parts 

 of the protein structure. The DPNH in the GPD complex may be 

 well shielded, and the absence of enhancement may be the result 

 of specific grouji interaction on the protein. That such an interaction 

 occurs is indicated by the appearance of a 365 m^A absorption band 

 in the GPD-DPN complex. 



