Clinical significance

1. oxalic acid separates from urine as insoluble “Ca-oxalate” crystals which can be seen microscopically in centrifuged deposits of urine. If passed in excessive amounts can form “ urinary calculus” (stones) in genitourinary tract.

2. Increase – the oxalic acid content of urine is increased – a) in Diabetes Mellitus; b) In certain liver diseases and in various conditions involving deficient tissue oxidation

3. Primary hyperoxaluria – a metabolic disease characterized biochemically by a continuous high urinary excretion of urinary oxalates

VI. Aminoacids

Normal value: Small amounts of aminoacids, both free and combined, are excreted in the urine.

The free aminoacid amounts to about 1.4mg/kg for women.

Age and Sex: Normal adult women excrete more aminoacids. There is some evidence for the existence of individual patterns of aminoacids excretion. One person tends to excrete more of one aminoacid and less of another than does another person.

The sex difference does not hold for children, who excrete somewhat less than adults; infants and prematurely born babies excrete about four times as much per kg as do older children.

Growing children excrete higher levels of hydroxyproline than do adults, reflecting an increased metabolism of collagen.

Aminoacidurias

Increased excretion of aminoacids than normal.

Types – Aminoacidurias can be divided into two groups.

(a) Over flow aminoaciduria – there is some metabolic defect, as a result there occurs an increase in plasma level of one or more aminoacids which exceeds the capacity of normal renal tubules to reabsorb them.

(b) Renal aminoaciduria – in this the plasma concentration of aminoacids is normal but because of defects in renal tubular reabsorption of aminoacids, an increase amount of one, several or all aminoacids escape in urine.

(a) “Overflow” Aminoacidurias

The generalised aminoaciduria found:

(i) In severe liver diseases like acute yellow atrophy and sometime in cirrhosis liver, leucine and tyrosine are excreted in urine. Tyrosine crystallizes in sheaves or tufts of fine needls; leucine in spherical shaped crystals, yellowish in color and with radial and circular striation. Both are insoluble in acetone and ether; but soluble in acids and alkalies.

(ii) Similarly aminoacidurias occur in wasting diseases.

(iii) Metabolic defects affecting a simple aminoacid or a small group include inherited disorders like phenyl ketonuria, maple-syrup disease, histidinaemia, histidinuria etc.

(b) Renal Aminoaciduria

The defect may be:

(i) Specific to one reabsorption mechanism as in cystinuria, in which there is failure to reabsorb cys, lys, arg and ornithine (a common transport defect) and in Hartnup disease, in which there is failure of reabsorption of all monoamino-monocarboxylic acids.

(ii) Nonspecific: as generalised aminoaciduria seen:

(a) In Fanconi syndrome, in which there is also failure to reabsorb glucose, phosphates, ammonia and other organic acids.

(b) In Wilson’s disease in which in addition to aminoaciduria (Ala, Asp, Glutamic acid) there is associated glucosuria, uric acid and phosphate excretion.

(c) An increase number of aminoacids are excreted by patients suffering from muscular dystrophies and by their mothers and siblings. Among these are Methionine or valine, Isoleucine or leucine, methionine sulfoxide or sarcosine methyl histidine.

(d) A generalised hyperaminoaciduria commonly occurs in lead intoxication and in persons exposed to other heavy metals like Hg, Cd and Uranium. Aminoaciduria in these cases appear to be due to impaired tubular reabsorption.

(e) More than 1.0 gm of peptides is excreted daily by the average normal human adult, which accounts for about ½ the urinary amino N2 and about 2% of total N.

(f) Large quantities of β-amino isobutyric acid (BAIB) are excreted in the urine of a small proportion (about 5% otherwise normal people). It has also been found in various pathologic states, sometimes with other aminoacids. It is apparently due to disturbance in the metabolism of thymine and dihydrothymine, which are precursors of this aminoacid.