Saposin A, a heat-stable 16-kDa glycoprotein, was isolated from Gaucher disease spleen and purified to homogeneity. Chemical sequencing from its amino terminus and of peptides obtained by digestion with protease from Staphylococcus aureus strain V-8 demonstrated that saposin A is derived from proteolytic processing of domain 1 of its precursor protein, prosaposin. Processing of prosaposin (70 kDa) also generates three other previously reported saposin proteins, B, C, and D, from its second, third, and fourth domains. Similar to saposin C, saposin A stimulates the hydrolysis of 4-methylumbelliferyl ,B-glucoside and glucocerebroside by ,B-glucosylceramidase and of galactocerebroside by ,B-galactosylceramidase, mainly by increasing the maximal velocity of both reactions. Saposin A is as active as saposin C in these reactions. Saposin A has no significant effect on other sphingolipid and 4-methylumbelliferyl glycoside hydrolases tested. Saposin A has two potential glycosylation sites that appear to be glycosylated. After deglycosylation, saposin A had a subunit molecular mass of 10 kDa and was as active as native saposin A. However, reduction and alkylation abolished the activation. A three-dimensional model comparing saposins A and C reveals significant sequence homology between them, especially preservation of conserved acidic and basic residues in their middle regions. Each appears to possess a conformationally rigid hydrophobic pocket stabilized by three internal disulffide bridges, with amphipathic helical regions interrupted by helix breakers. (16). Unlike saposin B, saposin C appears to interact directly with both enzymes to stimulate activity (17). The primary structure of saposin C was determined by chemical sequencing of its amino acids (18,19) and by deducing its sequence from nucleotide sequencing of a cDNA encoding prosaposin, its precursor (20). Saposin C has been reported to be deficient in a single patient with a variant form of Gaucher disease (21).Recently, the complete nucleotide sequence of a cDNA encoding prosaposin, the precursor of saposins B and C, was elucidated (20). Prosaposin is a 511-amino acid glycoprotein, and examination of its amino acid sequence reveals four saposin-like domains, two of which are saposins B and C; these are flanked by two additional domains, which we call saposin A and saposin D. Each domain is approximately 80-amino acid residues long; has nearly identical placement of cysteine residues, glycosylation sites, and helical regions; and is flanked by proteolytic cleavage sites. Molecular models indicate that the proteins derived from each domain can fold to give rise to a conformationally rigid hydrophobic pocket held together by three disulfide bridges. Proteolytic cleavage of prosaposin at each domain boundary was predicted to give rise to four saposin proteins (20). We recently have isolated saposin D, the protein arising from domain four of prosaposin and demonstrated it to be a specific sphingomyelin phosphodiesterase (EC 3.1.4.12) activator (22). In this...

The reaction of α-tetralone with various internal acetylenes can be catalyzed by Ru(H)2(CO)(PPh3)3 and gives 1:1 addition products. Symmetrically substituted dialkyl- and diarylacetylenes gave an E/Z mixture of 1:1 coupling products in good yields. 1-Phenyl-1-butyne afforded all four possible regio- and stereoisomers. 1-Trimethylsilyl-1-propyne gave only E-isomer with C–C bond formation exclusively at the carbon atom substituted with the silyl group. Other internal acetylenes having a trimethylsilyl group also proceeded regioselectively although mixtures of stereoisomers were obtained.

(8). Saposin C acts differently than saposin B since it interacts with the above enzymes increasing their maximal velocity and decreasing their Michaelis constant (9-11), whereas saposin B binds lipid substrates solubilizing them for hydrolysis. A third activator protein termed sphingolipid activator protein 3, also known as GM2 activator, is a specific activator for the hydrolysis of ganglioside GM2 by GM2 ,-N-acetylgalactosaminidase (12,13). Sphingolipid activator protein 3 is genetically distinct and unrelated to the saposins studied here. The two remaining saposin proteins, saposins A and D, were discovered (16) in 1989 after the cDNA encoding them was analyzed. Studies of the proteolytic processing of saposins B and C showed that both are initially biosynthesized as a large molecular mass precursor (70 kDa) that, after proteolytic processing, generates the 12-kDa saposin proteins (14,15). This laboratory cloned a cDNA for saposins B and C and showed that a common precursor generates both polypeptides (16). O'Brien et al. (16) and Collard et al. (17) proposed that two additional activator proteins similar in structure to saposins B and C are generated by proteolysis of the same precursor, which we call prosaposin. Furst et al. (18) obtained protein sequencing data on a polypeptide that they isolated. After comparison with nucle-

Novel transformations of metal porphyrins, such as nickel, zinc, copper, and silver porphyrins, to magnesium porphyrins are described. The reactions were accomplished by using 4‐methylphenylmagnesium bromide in toluene. The scope of the reactions was wide and various magnesium porphyrins were obtained (see figure). The products, acid‐sensitive magnesium porphyrins, were smoothly converted to free‐base porphyrins under weakly acidic conditions.

The palladium-catalyzed silylation of aryl chlorides with silylsilatranes proceeds under activator-free conditions; hence, wide functional group compatibility is displayed and boryl and siloxy groups are able to survive. Experimental and computational studies revealed that smooth transmetalation from the silylsilatrane to the arylpalladium chloride is facilitated by strong interaction between the Lewis acidic silicon and the chloride.