Re: Feathers for S excretion

[TomHopp@aol.com]

David Marjanovic wrote:

<H2S does occur as an end product of sulfur metabolism.>
In bacteria, yes. In vertebrates? News to me.

<Sulfate is indeed produced, and Reichholf knows this. I quoted last time, he 
writes in his 1998 paper:

... The question remains of what is "cheaper" ... physiologically less 
demanding, sulfate production or excretion through the skin in the form of 
feather keratin?>

Sulfate oxidation would take about 2 ATP per oxygen or 8 ATP. Making a single 
keratin protein takes hundreds of ATP, ignoring the growth and death of 
millions of feather-forming cells, for which the number of ATP ought to be in 
the billions or trillions, at least. All that for an alternative to the 
kidneys, which are already there for free, and which secret sulfate passively 
(0 ATP).


<recent observations on the deposition of harmful substances in bird plumage 
could be looked at in new light>

Deposition is a tricky word. Mercury poisoning leads to detectable mercury in 
the hair of victims, but no one has suggested there is a specific metabolic 
pathway involved. The mercury is everywhere, so some gets into hair -- while 
you are busy dying in convulsiosn, that is.

The following is really thick, but I think I get the drift:

<What would such a view require in advance? Firstly surely that the diet 
would have to include, at

least part of the time, a protein content exceeding the needs, protein that 
would additionally have to be rich in sulfur-containing amino acids. But a 
reduction of protein intake [eating less] to the needed level would have to

counteract another need that would have to be satisfied and could not be 
measurably lowered. [...] [Means, eating less protein also means eating less 
fat and carbohydrates, but these two are needed in large amounts, so eating

less protein is impossible.]" Burning protein yields, in contrast to fat and 
carbohydrates, end products that must be actively excreted. I don't really 
understand what the next few sentences imply: "In birds this is (the badly

water-soluble) ureic acid. Its excretion saves water, but all the more it 
concentrates the product of kidney excretion, especially when metabolism runs 
at full gear. The problematics of sulfur excretion results from this."

Probably this alludes to the fact that many sulfates (sodium sulfate...) are 
well water-soluble and thus can't be excreted without water, so HP Tom Hopp's 
statement "Sulfate can be concentrated 50 fold easily by the kidneys, nearly 
matching their 100 fold ability for urea nitrogen," may well apply to mammal 
kidneys, but probably not to bird kidneys.>

Sorry, not buying. Let me explain. You know that white stuff birds are so 
famous for pooping? Uric acid. Once it is concentrated by the kidneys, it 
moves to the cloaca where, as in the mammalian large intestine, solids are 
dehydreated by resorption of water. Viola! It crystallizes into an insoluble 
white powder. Predatory birds all poop white stuff, but herbivores -- like 
the yukky Canadian Geese ruining every park lawn in Seattle -- lay down 
little greenish brown dog-turds because they have a protein poor diet. Again, 
the body has incredibly efficient mechanisms for excreting wastes that are 
adaptable to different diets and meant to handle products such as sulfate. 
Reichholf's distinction between sulfate and urate solubility may be true, but 
it is not anywhere near as significant as he makes it out to be. Sulfate is 
one of the most soluble salts known, so only a few moles of water are needed 
to keep it moving. Even if it does precipitate, birds' deydration mechanism 
is cleverly designed to act AFTER the wastes have reached the final disposal 
site in the cloaca. Removal of every last water molecule is concievable, 
though, judging from the splatterability of most bird waste, total removal is 
never the goal. So keeping sulfate soluble is no problem.


<Reichholf goes on:>

But I'm about out of steam.


<Oh! Are there steroids ... that contain sulfur? I don't know any...>
taurocholic acid


<What is cystathione? . . . What exactly is homocystinuria?>
Check a basic biochemistry textbook like Lehninger, which was in use when I 
taught medical students about a million years ago.


<<The kidney has an awesome capacity to adapt.> But, apparently, no capacity 
to excrete sulfate without water.>

The cloacal resorption of water makes this irrelevant, along with (I'm sorry 
to say) Reichholf's hypothesis.

Thomas P. Hopp, former Cornell Medical College TA and
Author of DINOSAUR WARS, a science fiction novel published by iUniverse
Now Humans are the Endangered Species!  http://members.aol.com/dinosaurwars