Introduction to Human Nutrition

56 Introduction to Human Nutrition

Sources of nonspecifi c nitrogen
for humans

In earlier texts it would have been stated that, given
a suffi cient intake of the indispensable amino acids,
all that is then additionally needed to support body
protein and nitrogen metabolism would be a source
of “nonspecifi c” nitrogen (NSN) and that this could
be in the form of a simple nitrogen-containing
mixture, such as urea and diammonium citrate.
However, this is no longer a suffi cient description of
what is actually required to sustain an adequate state
of protein nutriture in the human. This can be illus-
trated by considering the nitrogen cycle, on which all
life ultimately depends (Figure 4.4). From this it can
be seen that some organisms are capable of fi xing
atmospheric nitrogen into ammonia, and plants
are able to use either the ammonia or soluble nitrates
(which are reduced to ammonia) produced by
nitrifying bacteria. However, vertebrates, including
humans, must obtain dietary nitrogen in the form
of amino acids or other organic compounds, possibly

as urea and purine and pyrimidines. Glutamate

and glutamine provide a critical entry of the

ammonia from the nitrogen cycle into other amino

acids. It is, therefore, important to examine briefl y

the way in which the human body may obtain this

NSN so as to maintain the nitrogen economy of the

individual.

Ammonia can be introduced into amino acids by

ubiquitous glutamate ammonia ligase (glutamine

synthetase) that catalyzes the following reaction:

Glutamate + NH+ 4 + ATP

→ Glutamine + ADP + Pi + H+ (4.1)

and (2) via the glutamate dehydrogenase reaction:

α-Ketoglutarate + NH+ 4 + NADPH

∫^ L-Glutamate + NADP + H 2 O (4.2)

However, because Km for NH+ 4 in this reaction is high

(>1 mM), this reaction is thought to make only a

modest contribution to net ammonia assimilation in

the mammal.

Reduction by

some anaerobic

bacteria

Synthesis in plants

and microorganisms

Nitrate

NO– 3

Degradation by

animals and

microorganisms

Denitrification Nitrogen fixation

by some bacteria

(e.g., Klebsiella,

Azotobacter, Rhizobium)

Nitrification

by soil bacteria

(e.g., Nitrobacter)

Nitrification

by soil bacteria

(e.g., Nitrosomonas)

Ammonia

Nitrite

NO– 2

Amino acids

and other

reduced

nitrogen–carbon

compounds

NH 4

Figure 4.4 The nitrogen cycle. The most abundant form of nitrogen is present in air, which is four-fi fths molecular nitrogen (N 2 ). The total amount
of nitrogen that is fi xed in the biosphere exceeds 10^11 kg annually. Reproduced from Lehninger AL, Nelson DL, Cox MM. Principles of Biochemistry,
2nd edn. New York: Worth, 1993.