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Re: [APD] Nitrite toxicity and GH
Stuart Halliday responded with a link to the published computation from PK:
The whole concept is a bit odd, which strikes me as strange coming from PK.
Although certainly not infallible, there are folks on the PK staff whom I
respect highly. But one clue that a basic misunderstanding is involved
comes from the site's brief description of nitrite toxicity which implies a
link between gill membrane transport of nitrite and its toxicity. So far as
I know, there is no relationship there. Active transport and passive
(solely concentration driven) transport are complex subjects, but regardless
of the transport technique into the fish's body fluids, the toxicity of
nitrite in fish is entirely a binding site competition on the hemoglobin
molecule. The chloride ion does not "detoxify" the nitrite; it blocks its
stable bonding to the oxygen-carrying molecule (which binding blocks O2
transport). The use of "detoxify" implies a change in the nitrite ion
itself, and there is none. Toxicity inhibition does not mean that the
nitrite is no longer toxic, only that the toxicity is no longer expressed or
observed, it effect is inhibited, but the nitrite itself is unchanged.
There are other somewhat bizarre questions raised by the brief note of the
computation. Nitrite (and many other) toxicities are generally
concentration-dependent. The total mass in a system is not material. This
raises the question of what exactly is the function or use of the volume
factor. Volume is trivial, concentration matters.
The basic question I had originally was on the import of the GH measure. By
the way, GH is again a concentration factor, not a total mass factor. GH,
as measured by hobby test kits, is a measure of the sum of the calcium
(Ca++) and magnesium (Mg++) ions in the solution. In reality it is
measuring all of the periodic table group known as the alkaline earth
metals, but in potable (and suitable for fish-keeping) waters, this is
almost entirely the calcium and magnesium - the others in the group are
uncommon to rare in FW. Their percentage concentration will be trivial
compared to the two primary elements/ions measured by us. Neither chloride
nor nitrite ions are detected or measured by that test. Both chloride and
nitrite are anions, negatively charged ions, while the calcium and magnesium
ions are cations - positively charged ions - thus the two cannot be involved
in blocking the toxicity of an anion (nitrite). Higher level of GH are
commonly associated with higher pH, but that also is no related to the
Ca++/Mg++ themselves, but to the anions with which those cations are
commonly associated in FW and marine systems - specifically those
pH-affecting anions are the bicarbonate (HCO3) and carbonate (CO3) ions. So
what is the significance of the GH? To me it quite like the volume factor -
what is the relationship and its import?
Perhaps there was an original article on the topic, or some external
reference, which justifies the volume and GH relationship. But if there is
such, I would love to see it. If for no other reason than that, to
aquarists, chloride ion is common as dirt, but is totally unquantifiable.
Any technique that aids measuring of chloride ion (which is within the reach
of the hobbyist with no more than simple hobby test kits) would be a boon.
Apologies for the extended length of this reply, and all of the
"chem.-speak" and "physio-speak", but they are at the heart of the problem
to me. Without clarification or reference, the suggested relationship seems
to be wishful thinking.
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