is an oily fluid, strongly alkaline, soluble in alcohol, ether, and water. It boils at 9° C.

Naphthylamine (C10H,N) is a product of the oxidation of proteids, and naphthalene (C1oHg) has been detected by Hoppe-Seyler in the

urine.

Amides are derivatives of acids which have exchanged the hydroxyl (HO) of the acid for amidogen (NH). Urea (CON,H4) is a typical member of the group. By some it is regarded as the diamide of carbonic acid. Hydrogen carbonate has the formula CO(OH)2; replace the hydroxyls by amidogen, and we get CO(NH2)2. From another point of view it may be regarded as being built in the type of two molecules of ammonia, in which two hydrogen atoms are replaced by the dyad radicle CO :—

Urea is isomeric with ammonium cyanate (NH)CNO, from which it was first prepared synthetically by Wöhler (1828). When ammonium cyanate is heated to 100°C., the atoms rearrange themselves to form urea. It may also be prepared by the action of ammonia on carbonyl chloride (COCl2 + 4NH3=CON2H,+2NH,CI).

By uniting with water, urea forms ammonium carbonate. This it does under the influence of a specific organised ferment (micrococcus urea) in decomposing urine (CON2H, +2H2O=(NH4)2CO3).

Urea is met with in nearly all the solids and fluids of the body, but chiefly in the urine; about 30 grammes (500 grains) are on the average excreted by the kidneys of an adult daily. Urea is the chief end product of the metabolism of the nitrogenous constituents of the body.

Oxaluric acid is urea in which one atom of hydrogen is replaced by the radicle of oxalic acid (i.e. oxalic acid minus HO).

The Amido-Acids are compounds which show partly the character of an acid, and partly that of a weak base. They may be considered as ammonias, in which one or more atoms of hydrogen are replaced by radicles of an acid, thus resembling alkaloids; or, on the other hand, as acids in which one or more hydrogen atoms of the acid radicle are replaced by amidogen (NH). The principal are as follows:

:

(a) Glycocine.—This is also known as glycine, glycocoll, and amidoacetic acid, the last-mentioned name expressing its constitution. The formula for acetic acid is CHO,; if one atom of hydrogen be re

2

G

placed by NH2, we obtain C2H3(NH2)O, C2H NO2, which is the
It has been obtained
formula for glycocine or amido-acetic acid.
synthetically by the action of monochloracetic acid on ammonia

FIG. 31.-Glycocine crystals.

(C2H ̧CIO2+NH2=HCl+C2H2(NH2)O2). In a pure state glycocine crystallises in rhombohedric prisms, soluble in water, but not in ether or alcohol. The aqueous solution is acid.

When heated on platinum foil, it leaves a colourless residue, which, on warming with a drop of caustic soda solution, forms an oily drop which runs about without touching the surface of the platinum. (Scherer's test.) When heated in a glass tube open at both ends, it sublimes, and a smell of amylamine is given off.

Glycocine is found in combination in glycocholic acid (one of the bile acids), and in hippuric acid (in the urine). It is a product of decomposition of proteids, and also of gelatin, mucin, and other albuminoids. It occurs free in small quantities in the intestine as a result of the decomposition of the bile that occurs there. It is probably largely reabsorbed as such. Part may be transformed into urea.

(b) Leucine (CH,,NO2) is amido-caproic acid, i.e. caproic acid (CH12O2) in which an H is replaced by NH, or, according to some, oxy-caproic or leucic acid (C6H12O3), in which hydroxyl (HO) is replaced by NH.

Leucine forms yellowish-brown spheres consisting of masses of needle-shaped crystals, soluble in water and slightly soluble in alcohol, but insoluble in ether. By heat it is decomposed into carbonic acid and amylamine (C6H13NO2=CO2+CH11.NH2); by hydriodic acid into caproic acid and ammonia (C6H13NO2+H2=CH12O2+NH3); with sulphuric acid it yields ammonia and valerianic acid; with potassium permanganate, oxalic acid, carbonic acid, valerianic acid and ammonia. Probably similar decompositions occur in the body. Probably also leucine is one of the intermediate products in the formation of urea.

Leucine is most important as one of the chief decomposition products of proteids, and is formed when proteids are decomposed with strong acids or alkalis, or undergo putrefaction, and within the body by the activity of certain ferments, especially of one secreted by the pancreas called trypsin. It is found in small quantities as a constituent of many organs and tissues, particularly of the pancreas. In certain cases, however (spleen, thymus, &c.), it appears to be formed by post-mortem changes, and not to be a constituent of the healthy living tissue. It

may be separated from an organ by making a watery extract; this is

boiled, acidified and filtered to

separate any proteid; to the filtrate basic lead acetate is added, and again it is filtered; excess of lead is removed from the filtrate by a stream of sulphuretted hydrogen. The lead sulphide is filtered off, the filtrate is evaporated to a syrup, and extracted with hot alcohol. The alcoholic extract is evaporated, and the residue is leucine.

(e) Tyrosine C,H11NO, is amido - oxy-phenyl-propionic

acid.

FIG. 32.-Leucine crystals.

Propionic acid has the formula C3H6O2. Amido-propionic acid CH(NH)O2 is also called alanine. Oxyphenyl-propionic acid is propionic acid in which one H is replaced by oxyphenyl (C,H,.OH), i.e. C2H(CH4.OH)O,; if another H in this be replaced by NH, we get C2H,(NH2)(CH1.OH)O2=C,H11NO3, which is amido-oxyphenyl-propionic acid or tyrosine.

To separate tyrosine from

an albuminous fluid, the fluid must be boiled and the precipitated proteid filtered off. The filtrate is evaporated to a third of its volume on the waterbath, precipitated with lead acetate and filtered, a stream of sulphuretted hydrogen is passed through the filtrate, and the lead sulphide so precipitated, filtered off; the filtrate is evaporated, and crystals of tyrosine sepa

rate out.

Tyrosine crystallises in slender needles, often in groups, slightly soluble in water, but insoluble

a

in alcohol and ether. On oxidation it yields benzoic aldehyde, hydro

cyanic, benzoic, acetic, formic, and carbonic acids.

Tests. i. Heat in a watch-glass with concentrated sulphuric acid; cool; add water and a few pieces of chalk; there will be an effervescence; filter; evaporate to a small bulk; add a few drops of a neutral solution of ferric chloride; a violet colour is produced (Piria).1 ii. Millon's reagent gives a red solution, the tint of which deepens on heating (Hoffmann).2 Tyrosine is generally found along with leucine, and like it results from the decomposition of proteids.

11

(d) Amido-valerianic acid (C,H,,NO2) is a product of decomposition of proteids, but it only occurs in small quantities in comparison with leucine and tyrosine.

(e) Sarcosine C,H,NO, is methyl-glycocine, i.e. amidoacetic acid in which one H is replaced by methyl C,H,(CH3)(NH2)O2. It is not found in the body, but is a product of decomposition of creatine.

(f) Creatine C,H,NO2. Sarcosine (C3H-NO2) united to cyanamide CN.NH2 gives creatine. When creatine is boiled with baryta water, it takes up water and yields sarcosine and urea

(C1H,N ̧O2+H2O=C2H2NO2+CON2H1).

3 2

[creatine]

[sarcosine]

[urea]

Creatine crystallises with one molecule of water of crystallisation in colourless rhombohedric prisms, soluble in water, almost insoluble

2

(g) Creatinine C,H,N3O is creatine minus H2O. It can be formed by heating the latter with boiling water for a long time; or more readily by heating it with hydrochloric acid. When creatinine is heated with baryta water it yields methylhydantoin (C4H6N.O) and ammonia (C,H,N,O+H,O=C,H,N2O2+NH3). It crystallises in large colourless prisms soluble in water and alcohol, but almost insoluble in ether. It has an alkaline taste and reaction. Salkowski 3 has, however, recently stated that the greater part of this alkalinity is due to adherent impurities.-Tests. i. With zinc-chloride it gives a characteristic crystalline precipitate (groups of fine needles). This

1 Ann. Chem. Pharm. vol. lxxxii. p. 252.

2 Ibid. vol. lxxxvii. p. 123. See also L. Meyer, ibid. vol. cxxxii. p. 156.
3 Zeit. physiol. Chem. xii. 211.

consists of a combination of

zinc-chloride with creatinine (C,H-N3O.ZnCl2). This test is used for the quantitative estimation of

creatinine. ii. A solution of creatinine,

as in urine, acidulated by nitric acid gives with phosphomolybdic acid a yellow crystalline precipitate soluble in hot nitric acid. iii. It reduces an alkaline solution of cupric hydrate such Fehling's solution.

as

(h) Taurine C2H,NSO, is amidoisethionic acid. Isethionic acid is sulphurous acid in which an atom of hydrogen is replaced by the monatomic radicle oxy-ethylene (C2H.OH). If the hydroxyl of this radicle be replaced by NH, we obtain taurine. The fol

2

FIG. 35.-Creatinine crystals.

lowing formula will assist in the understanding of this relationship : H2SO3, Sulphurous acid.

H(C2H,.OH)SO3, Isethionic acid.

H(C2H1.NH2)SO3=C¿H¬NSO3, Taurine.

Taurine is artificially prepared by heating ammonium isethionate (C,H,NSO4) which parts with H2O and so yields taurine.

Cyanic acid with taurine forms tauro-carbamic acid, in which form taurine is partially excreted in the urine

(CNHO+C2H,NSO3=C2H ̧N2SO4.

of this acid that occurs in the intestine; the greater amount that