Agriculture Reference
In-Depth Information
fIRst cAme nItRogen
Claude Berthollet's report to the French Academy of Sciences in 1785 that a gas
emitted by decomposing animal tissue (ammonia) contained 83% nitrogen and 17%
hydrogen (by weight) provided an early clue to the presence of nitrogen in protein.
Research by others found no nitrogen in sugars, starches, or fats, but modest amounts
were present in wheat flour, which was thus credited with containing “animal mat-
ter.” Since it was known that air contained nitrogen, there was speculation whether
the high amounts in animal tissue were derived from this source. François Magendie
fed single foods, like sugar or olive oil (thought to be nutritious but containing no
nitrogen) to dogs, and the dogs died. When dogs were fed bread (containing some
nitrogen), they lived longer but died nevertheless. Likewise, he reported in 1841 that
gelatin obtained by boiling bones was not a complete food for dogs, even though it
was high in nitrogen. As further research would reveal, Magendie's diets had several
nutrient deficiencies, but the quantity and quality (identity and proportions of indis-
pensable amino acids) of dietary protein were certainly limited, and there was no
evidence that nitrogen in air helped at all.
Coincident with and relevant to the studies of Magendie, Jean Baptiste Boussingault
reported in 1836 that cereal crops were unable to use atmospheric nitrogen to sup-
port their own growth but legumes could, and he proposed that the relative nutrient
value of plants could be assessed from their nitrogen content. He also reported in
1839 on nitrogen balance trials with a horse and a dairy cow in which he measured
nitrogen intake from their food and nitrogen excreted in feces, urine, and (in the case
of the dairy cow) milk. J. B. Dumas, a contemporary of Boussingault, concluded in
1841 that only plants could synthesize the nitrogenous substances found in animals,
a statement found to be false when it was discovered that dispensable amino acids
can be synthesized by animal tissues and incorporated into their nitrogenous sub-
stances. However, clarification of this issue required further research. These nitrog-
enous substances were ultimately named proteins, from the Greek word
prōte
, for
primary, referring to their role as primary components of the animal body. Several
of these animal substances were characterized and given names, such as albumin,
fibrin, and casein. Although they differed in physical properties, they were found to
contain about 16% nitrogen. Liebig analyzed animal muscles and reportedly found
no carbohydrate or fat—only protein. Thus, he stated in his 1842 topic on application
of chemistry to physiology that protein is the only true nutrient because it serves as
the machinery of the body and provides the fuel for its work.
Carl Voit, Liebig's protégé, supervised a laboratory in Munich where nitrogen
balance procedures were used to estimate protein requirements. Wilbur Atwater
of Wesleyan University in Connecticut studied in Voit's laboratory and set a pro-
tein standard for American workmen of 125 g per day. Russell Chittenden of Yale
University challenged this high value in 1895 when he found that his own needs
did not exceed 40 g per day. Soon, the significance of amino acids became appar-
ent with the discovery of tyrosine and leucine—released when proteins were boiled
with strong acids. Further, their connection to the physiology of digestion was estab-
lished when these same amino acids were found when pepsin-treated proteins were
exposed to pancreatic trypsin. In 1902, F. G. Hopkins and S. W. Cole at Cambridge
