Re: Why does mammalian liver contain more catalase than bird and fish liver?l

Daniel:
     
     I think this is a great question.  You have a hypothesis, a question 
and have decided to test it.  I did some horseradish peroxidase 
experiments when I was younger, (A different enzyme that catalyzes the 
same reaction).  Actually, I do not think I can answer your question with 
the information you have given me.  In enzymology, which is the study of 
enzymes, it turns out that just knowing which enzyme produces more of 
substance/product X is not indicitive of quantity, especially when we 
don't control for time.  If one enzyme, say bird was 100 times as active 
but there was 100 times less, your experiment would still show that it had 
the same amount, which is clearly not true. 
     I really like the fact that you repeated each experiment five times, 
but we also need to know what all five numbers were.  Is the result you 
gave me an average?  We also need to know your error, as well as the 
standard deviation for your data.  For example, if the numbers for lamb 
and bird vary around 26 and 24cc, those are probably in fact the same 
number, and there is no difference between lamb and bird.  If your values 
vary widely, it might be possible within your ability to measure the 
enzyme, there is no difference bewteen any of them.  
     The next factor you did not tell me about was time, and this relates 
to how efficient one enzyme might be over another at converting peroxide 
H2O2 to water and oxygen.  Did you let all of them react for the same 
amount of time, or until you saw no more bubbles, or how?  If you let all 
of them go until completion, which is when the enzyme ceased to function, 
or you ran out of peroxide for example, then we can essentially compare 
one volume of oxygen to another as a means of relatively quantitating.  If 
they all went for 2 minutes, and some were still bubbling that doesn't 
necessarily measure amount of catalase, but rather a combination of how 
much catalase and how good it is.  We usually speak of enzymatic activity 
in terms of units of substrate per unit time... In this case you could say 
Lamb liver produced 2.6cm3 oxygen per minute.
     The third variable is enzyme conditions.  Enzymes are extremely pH 
and temperature dependant, since they are proteins.  Most enzymes are most 
active at the normal temperature of the host, so it is possible that the 
fish enzyme was performing out of it's range, and was therefore less 
active than it 'should be.'  It is a good idea to keep temerpature 
constant when doing these experiments.
    And finally, I think your thoughts on why these results exist are 
good.  Actually, the great part of doing experiments is that there are no 
wrong answers.  Knowing that all three might contain the same amount of 
catalase is pretty neat too.  In truth, I would guess the warm-blooded 
animals might have more catalase since their metabolisms would tend to 
produce more reactive oxygen species (Like hydrogen peroxide) requiring 
catalase.   
     Whew, that was a mouthful.  I hope it helps.
-Matt-