5. ONIUM COMPOUNDS
197
recent work by Cromwell and Rennie (53) carries the implication that
betaine is formed in the leaf tissue from choline, by oxidation, and that
exposure to light stimulates this conversion, perhaps through an increase
in the intracellular oxygen content. At all events, betaine is utilized
quite slowly by the plant, and thus continues to accumulate, reaching
high concentration in older leaves. The accumulation of this material in relatively high concentration raises the question as to its function in the plant tissue. Betaine is known to participate in transmethylation reactions in vertebrates where it can be utilized for the biosynthesis of methionine (54, 55). Whether this reaction occurs also in
plants and more particularly in leaves is not known. Very little is
known about the participation of betaine in other pathways of metabolism, but teleologically it might well be assumed that betaine fulfills
some important function in the physiology of the plant.
In certain anaerobic bacteria, grown on betaine as a sole carbon
source, betaine is transformed to dimethylglycine, while the methyl
group which is formed in this reaction is utilized (possibly after oxidation) for the biosynthesis of acetate (56). Detailed formulation of this
reaction would be of interest for it might disclose a new pathway for
the utilization of the energy of the onium bond in betaine for biosynthetic purposes.
3. Carnitine, Crotonobetaine, and y-Butyrobetaine
Although carnitine was discovered over 50 years ago (57, 58) as a
constituent of skeletal muscle the first clue as to its biological role
came only in 1952 when it was recognized as identical with a vitamin,
B T (59), SL growth factor essential for the meal worm Tenebrio molitor
and certain other insects. A great deal of information is available on
the natural distribution of carnitine, and the subject was competently
reviewed very recently by Fraenkel and Friedman (60) whose paper
should be consulted also for the chemistry of this compound and for a
discussion of the biological effects of carnitine and its analogs. Carnitine is almost universally distributed, some exceptions being E. colt, the
protozoan Tetrahymena galeii, and the hen's egg before development of
the embryo. The tissues with the highest content are skeletal muscle
and related tissues, such as the electric organ of the torpedo (61),
which is a modified muscle end-plate.
In spite of the great advance represented by the recognition of the
vitamin-like properties of carnitine, little is known about its biochemical
solutions of choline or betaine aldehyde, the implication being that the choline oxidase system is present in the leaves.
197
recent work by Cromwell and Rennie (53) carries the implication that
betaine is formed in the leaf tissue from choline, by oxidation, and that
exposure to light stimulates this conversion, perhaps through an increase
in the intracellular oxygen content. At all events, betaine is utilized
quite slowly by the plant, and thus continues to accumulate, reaching
high concentration in older leaves. The accumulation of this material in relatively high concentration raises the question as to its function in the plant tissue. Betaine is known to participate in transmethylation reactions in vertebrates where it can be utilized for the biosynthesis of methionine (54, 55). Whether this reaction occurs also in
plants and more particularly in leaves is not known. Very little is
known about the participation of betaine in other pathways of metabolism, but teleologically it might well be assumed that betaine fulfills
some important function in the physiology of the plant.
In certain anaerobic bacteria, grown on betaine as a sole carbon
source, betaine is transformed to dimethylglycine, while the methyl
group which is formed in this reaction is utilized (possibly after oxidation) for the biosynthesis of acetate (56). Detailed formulation of this
reaction would be of interest for it might disclose a new pathway for
the utilization of the energy of the onium bond in betaine for biosynthetic purposes.
3. Carnitine, Crotonobetaine, and y-Butyrobetaine
Although carnitine was discovered over 50 years ago (57, 58) as a
constituent of skeletal muscle the first clue as to its biological role
came only in 1952 when it was recognized as identical with a vitamin,
B T (59), SL growth factor essential for the meal worm Tenebrio molitor
and certain other insects. A great deal of information is available on
the natural distribution of carnitine, and the subject was competently
reviewed very recently by Fraenkel and Friedman (60) whose paper
should be consulted also for the chemistry of this compound and for a
discussion of the biological effects of carnitine and its analogs. Carnitine is almost universally distributed, some exceptions being E. colt, the
protozoan Tetrahymena galeii, and the hen's egg before development of
the embryo. The tissues with the highest content are skeletal muscle
and related tissues, such as the electric organ of the torpedo (61),
which is a modified muscle end-plate.
In spite of the great advance represented by the recognition of the
vitamin-like properties of carnitine, little is known about its biochemical
solutions of choline or betaine aldehyde, the implication being that the choline oxidase system is present in the leaves.