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The position of mevalonic acid and isopentenyl pyrophosphate (3 methylbut 3 enyl pyrophosphate) as key intermediates in the biosynthesis of isoprenoid compounds is now firmly established, as also is the route by which mevalonic acid is converted into isopentenyl pyrophosphate. This transformation occurs through three ATP-dependent steps with the intermediate formation of mevalonate 5-phosphate and mevalonate 5-pyrophosphate (Tchen, 1957, 1958; de Waard & Popjbk, 1959; Chaykin, Law, Phillips, Tchen & Bloch, 1958; Bloch, Chaykin, Phillips & de Waard, 1959; Henning, Moslein & Lynen, 1959; de Waard, Phillips & Bloch, 1959). It has also been demonstrated by Agranoff, Eggerer, Henning & Lynen (1960) that the isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate by isopentenyl pyrophosphate isomerase is an essential initial step in the conversion of isopentenyl pyrophosphate into a number of trans-isoprenoid compounds. Because of the ease with which the pyrophosphate anion is eliminated from an allylic compound such as dimethylallyl pyrophosphate, the latter behaves as an electrophilic reagent capable of attacking the methylene group of isopentenyl pyrophosphate, with regeneration of an allylic pyrophosphate end group. Several workers (Park & Bonner, 1958; Kekwick et al. 1959; Lynen & Henning, 1960; Archer, Ayrey, Cockbain & McSweeney, 1961) have shown that mevalonate and isopentenyl pyrophosphate are incorporated into high-molecular-weight rubber by freshly tapped Hevea brasiliens8i latex, and it has been assumed (Lynen & Henning, 1960) that isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate is again a necessary initial step in the formation of the polyisoprene chain. However, experimental evidence for this view with respect to rubber biosynthesis is still required, particularly since it is not known whether rubber possesses a terminal isopropylidene group (as would be expected if the polyisoprene chain were initiated from dimethylallyl pyrophosphate). No work has been reported, as yet, on the effect of the different cytological components of latex on the incorporation of mevalonate or isopentenyl pyrophosphate into rubber. In addition to rubber particles, aqueous serum and a small proportion of microsomes and mitochondria, freshly tapped Hevea latex contains specialized particles, the most abundant of which are spherical bodies of about 3,u diameter, bounded by an osmotically sensitive membrane and possessing a liquid interior in which other material may be suspended (Ruinen, 1950; Schoon & Phoa, 1956). These spherical bodies have been named 'lutoids' by Homans & van Gils (1948). A detailed cytological investigation of Hevea braailiensis latex and latex vessels has been carried out by P. B. Dickenson (unpublished work). In the present paper studies on the incorporation into rubber of mevalonate and isopentenyl pyrophosphate by the three components rubber, aqueous serum and lutoid particles are described.

The biosynthesis of rubber. Incorporation of mevalonate and isopentenyl pyrophosphate into rubber by Hevea brasiliensis-latex fractions