96
FRED H. WILT
of polysomes to Hb synthesis may not be simple, however, for the rate
of restoration of Hb synthesis after washing was much faster than
recovery of polysome structure; the absolute necessity of polysomes for
Hb synthesis is open to question. In this connection, Lamfrom and
Knopf (1964) have shown the 80S ribosomes of reticulocyte lysates can
initiate globin chain synthesis. The 80 S ribosomes were concentrated by
centrifugation prior to testing their ability to form Hb, and it is possible
that some polysomes were formed during this centrifugation procedure.
Finally, let us turn for a moment to another factor involved in the
rate of the final assembly of the Hb molecule, the role of the prosthetic
group of Hb. It has been known for some time that the synthesis of
globin is tightly coupled to the availability of iron or heme (Kruh and
Borsook, 1956; Borsook et al., 1957). Recent findings have reinforced this
conclusion and demonstrated that heme may be necessary for maximal
rates of globin synthesis. Just how heme participates is unknown, but it
may be integrated with peptide-chain formation (Waxman and Rabinovitz, 1965b). Bruns and London (1965) have analyzed the effect of
hemin on Hb formation in intact reticulocytes from rabbits maintained
on a low iron diet. A twofold stimulation of Hb synthesis was rapidly
effected by 5 X 10~
5 M hemin. Karibian and London (1965) have also
shown that hemin inhibits formation of 8-aminolevulinic acid which is
the first distinctive precursor of heme. This could act as an effective
"feedback inhibition" mechanism for regulating the amount of heme
needed for hemoglobin synthesis.
Waxman and Rabinovitz (1965a, 1965b; Rabinovitz and Waxman,
1965) have examined the polysome profile of reticulocytes. Iron or hemin
preserved the polysome structures of reticulocytes incubated in vitro.
Omissions of these compounds, or chelation of iron with bipyridine, led
to disappearance of polysomes. In the presence of both iron (or hemin)
and exogenous amino acids, the rate of Hb synthesis was markedly
stimulated. However, the rate of Hb synthesis was not directly proportional to the polysome content.
Gribble and Schwartz (1965) have studied the effect of the heme
precursor, protoporphyrin IX, on Hb synthesis in a cell free system.
Protoporphyrin had a markedly stimulatory effect on the Hb synthesis,
and they showed that this was due to a stimulation of the release (to
the supernatant) of already complete ribosome-bound polypeptide chains.
Matioli and Eylar (1964) have discovered reticulocytes can synthesize
apoferritin. They measured relative rates of apoferritin and Hb synthesis
under various conditions and suggest a relation between the 2 types of
synthesis; it is proposed that apoferritin regulates inorganic iron levels
in the cell, which in turn affect Hb synthesis.
The importance of all the foregoing observations for the developmental
FRED H. WILT
of polysomes to Hb synthesis may not be simple, however, for the rate
of restoration of Hb synthesis after washing was much faster than
recovery of polysome structure; the absolute necessity of polysomes for
Hb synthesis is open to question. In this connection, Lamfrom and
Knopf (1964) have shown the 80S ribosomes of reticulocyte lysates can
initiate globin chain synthesis. The 80 S ribosomes were concentrated by
centrifugation prior to testing their ability to form Hb, and it is possible
that some polysomes were formed during this centrifugation procedure.
Finally, let us turn for a moment to another factor involved in the
rate of the final assembly of the Hb molecule, the role of the prosthetic
group of Hb. It has been known for some time that the synthesis of
globin is tightly coupled to the availability of iron or heme (Kruh and
Borsook, 1956; Borsook et al., 1957). Recent findings have reinforced this
conclusion and demonstrated that heme may be necessary for maximal
rates of globin synthesis. Just how heme participates is unknown, but it
may be integrated with peptide-chain formation (Waxman and Rabinovitz, 1965b). Bruns and London (1965) have analyzed the effect of
hemin on Hb formation in intact reticulocytes from rabbits maintained
on a low iron diet. A twofold stimulation of Hb synthesis was rapidly
effected by 5 X 10~
5 M hemin. Karibian and London (1965) have also
shown that hemin inhibits formation of 8-aminolevulinic acid which is
the first distinctive precursor of heme. This could act as an effective
"feedback inhibition" mechanism for regulating the amount of heme
needed for hemoglobin synthesis.
Waxman and Rabinovitz (1965a, 1965b; Rabinovitz and Waxman,
1965) have examined the polysome profile of reticulocytes. Iron or hemin
preserved the polysome structures of reticulocytes incubated in vitro.
Omissions of these compounds, or chelation of iron with bipyridine, led
to disappearance of polysomes. In the presence of both iron (or hemin)
and exogenous amino acids, the rate of Hb synthesis was markedly
stimulated. However, the rate of Hb synthesis was not directly proportional to the polysome content.
Gribble and Schwartz (1965) have studied the effect of the heme
precursor, protoporphyrin IX, on Hb synthesis in a cell free system.
Protoporphyrin had a markedly stimulatory effect on the Hb synthesis,
and they showed that this was due to a stimulation of the release (to
the supernatant) of already complete ribosome-bound polypeptide chains.
Matioli and Eylar (1964) have discovered reticulocytes can synthesize
apoferritin. They measured relative rates of apoferritin and Hb synthesis
under various conditions and suggest a relation between the 2 types of
synthesis; it is proposed that apoferritin regulates inorganic iron levels
in the cell, which in turn affect Hb synthesis.
The importance of all the foregoing observations for the developmental