Northerncardinal

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I'm a bit confused about how uric acid (pKa = 5.4) gets precipitated in acidic environments...the distal tubules and collecting ducts? Is it the deprotonated (charged) form of uric acid that ppts or the protonated form??

Uric acid is soluble at physiological pH. how?
 

atomheart

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The baseline solubility of uric acid is extremely low, and maximum is close to the amount that already exists in blood. At physiological pH it exists as urate salt which forms crystals as concentration increases. As crystals are formed they cannot be oxidized back to a more soluble form.

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Phloston

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I'm a bit confused about how uric acid (pKa = 5.4) gets precipitated in acidic environments...the distal tubules and collecting ducts? Is it the deprotonated (charged) form of uric acid that ppts or the protonated form??

Uric acid is soluble at physiological pH. how?
Am honoured you linked me here, thanks.

pKa of the important proton in uric acid is 5.8. Below that pH the molecule is uric acid and is not charged. At physiological pH (7-range) it mostly exists as the urate ion (single negative charge). The urate ion is more soluble than the uncharged uric acid form. So at lower pH, you get more uric acid, which is less soluble, and once solubility threshold is reached, precipitation of uric acid stones ensues.

From UpToDate:

"Uric acid solubility in the urine falls from approximately 200 mg/dL (1.2 mmol/L) at a urine pH of 7 (a setting in which 95 percent of uric acid is present as the more soluble urate anion) to 15 mg/dL (0.09 mmol/L) at a urine pH of 5 (a setting in which most of the uric acid is the less soluble, undissociated acid) [1,10].

The importance of a low urine pH was illustrated by the biochemical characterization of 341 uric acid stone formers from four studies. The mean urine pH in patients with uric acid nephrolithiasis was 5.4, and virtually all patients had a urine pH less than 6 [4-7]. "
 
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Northerncardinal

Northerncardinal

2+ Year Member
May 25, 2016
139
44
Status
Medical Student
Am honoured you linked me here, thanks.

pKa of the important proton in uric acid is 5.8. Below that pH the molecule is uric acid and is not charged. At physiological pH (7-range) it mostly exists as the urate ion (single negative charge). The urate ion is more soluble than the uncharged uric acid form. So at lower pH, you get more uric acid, which is less soluble, and once solubility threshold is reached, precipitation of uric acid stones ensues.

From UpToDate:

"Uric acid solubility in the urine falls from approximately 200 mg/dL (1.2 mmol/L) at a urine pH of 7 (a setting in which 95 percent of uric acid is present as the more soluble urate anion) to 15 mg/dL (0.09 mmol/L) at a urine pH of 5 (a setting in which most of the uric acid is the less soluble, undissociated acid) [1,10].

The importance of a low urine pH was illustrated by the biochemical characterization of 341 uric acid stone formers from four studies. The mean urine pH in patients with uric acid nephrolithiasis was 5.4, and virtually all patients had a urine pH less than 6 [4-7]. "
I understand the concept now. Thank you so much for your help! Appreciate it!
 
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