6-Fluoromevalonate blocks the incorporation of mevalonic acid, but not that of isopentenyl pyrophosphate, into non-saponifiable lipids in a rat liver multienzyme system. With 3H-labelled 6-fluoromevalonate, it was found that 6-fluoromevalonate is converted to its phospho and pyrophospho derivatives in this system. The kinetics of the two kinases were studied. 6-Fluoromevalonate 5-pyrophosphate is a potent competitive inhibitor of pyrophosphomevalonate decarboxylase (Ki 37 nM). In the multienzyme assay for cholesterol biosynthesis, there is accumulation of mevalonate 5-phosphate and mevalonate 5-pyrophosphate in the presence of 5 microM-6-fluoromevalonate, and 6-fluoromevalonate 5-pyrophosphate is more effective than 6-fluoromevalonate in inhibiting cholesterol biosynthesis. We suggest therefore that 6-fluoromevalonate blocks cholesterol biosynthesis at the level of pyrophosphomevalonate decarboxylase after being pyrophosphorylated.

The events leading to the formation of ␤-amyloid (␤A4) from its precursor (␤APP) involve proteolytic cleavages that produce the amino and carboxyl termini of ␤A4. The enzyme activities responsible for these cleavages have been termed ␤-and ␥-secretase, respectively, although these protease(s) have not been identified. Since ␤A4 is known to possess heterogeneity at both the amino and carboxyl termini, ␤-and ␥-secretases may actually be a collection of proteolytic activities or perhaps a single proteolytic enzyme with broad amino acid specificity.We investigated the role of cathepsin D in the processing of ␤APP since this enzyme has been widely proposed as a ␥-secretase candidate. 1, 2). The initial discovery of ␤A4 as a constituent in AD tissue prompted a number of investigations into the role of ␤-amyloid in this disease. The physical properties of ␤A4 combined with the genetic evidence linking mutations around the ␤A4 domain of the ␤-amyloid precursor protein (␤APP) to early onset forms of the disease (reviewed in Refs. 3 and 4) provide compelling evidence that this peptide is involved in the etiology of Alzheimer's disease. Unraveling the cellular mechanism of ␤APP processing and of ␤A4 production is, therefore, important in understanding the pathogenesis of AD. ␤A4 is formed by the proteolytic cleavage of a 695-770-amino acid integral membrane protein, the ␤APP at two distinct sites near the carboxyl terminus of this molecule. The proteolytic cleavage that generates the amino terminus of ␤A4 occurs approximately 28 residues extralumenal to the transmembrane domain of ␤APP, while that which generates the carboxyl terminus of ␤A4 takes place within the transmembrane domain. The protease activities that mediate these cleavage events are called ␤-and ␥-secretase, respectively. Cleavage at both the ␤-and ␥-secretase sites produces a soluble form of ␤A4 that is 40 amino acids long and ends at Val 40 (5); however, longer and potentially more pathogenic forms of ␤A4 that end at Ala 42 -Thr 43 , have also been identified (2, 6, 7). Whether these various forms of ␤A4 are produced by a single ␥-secretase is not known. A third major cleavage that is not involved in the formation of ␤A4 occurs within the ␤A4 domain of ␤APP and is generated by a protease activity called ␣-secretase.A number of recent studies have revealed important clues regarding the proteolytic processing of the ␤APP molecule. From these studies, two processing pathways have been proposed. The first is a constitutive secretion pathway that generates soluble forms of ␤APP, which are ultimately released by cells expressing the precursor protein (8). This pathway involves the primary proteolytic cleavage of ␤APP at the ␣-secretase site located within the ␤A4 domain (9, 10), releasing the large, soluble extracellular portion of ␤APP while simultaneously generating a 10-kDa membrane-associated ␤APP carboxyl-terminal fragment (11). This cleavage event precludes the formation of ␤A4. A minor cleavage event that also serves to liberate a soluble, albeit shorter...

From a side-by-side comparative study, the acyclic nucleoside phosphonates (R)-9-(2-phosphonylmethoxypropyl)adenine [(R)-PMPA] and 9-(2-methylidene-3-phosphonomethoxypropyl)guanine (MDL 74,968) proved more selective in their inhibitory effect on human immunodeficiency virus types 1 and 2, feline immunodeficiency virus, and Moloney murine sarcoma virus (MSV) in cell cultures than the 9-(2-phosphonylmethoxyethyl) derivatives of adenine (PMEA) and guanine (PMEG). In particular, PMEG proved quite toxic. PMEA, (R)-PMPA, and MDL 74,968 afforded a marked delay in MSV-induced tumor initiation in MSV-infected newborn NMRI mice and substantially delayed associated animal death at doses as low as 4 to 10 mg/kg of body weight. Treatment of the NMRI mice with PMEA, (R)-PMPA, and MDL 74,968 at 25 or 50 mg/kg resulted in a high percentage of long-term survivors.

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