Analysis of lactase processing in rabbit

Species differences in the sites of cleavage of pro-lactase to lactase supports lack of selective pressure

The pro-sequences in pro-lactase-phlorizin hydrolase (LPH) are needed for lactase to proceed past the ER, but are irrelevant as to the enzymatic activities. Hence, in all species removal of the pro- sequences (or most of them) must take place after the ER. Contrary to this, the details of the removal of these pro-sequences are to be expected to differ in the various species, since they are not subjected to selective pressure. Using site-directed mutagenesis we investigated processing in rabbit. The first cleavage occurs by furin (or furin-like PCs) and takes place at R-A-A-R(349) in the pro-sequence, generating the known 180 kDa intermediate. Replacing R(349) by Q results in a mutant which is not cleaved but nevertheless transported to the cell surface as demonstrated by immunofluorescence. Further processing of either the 180 kDa intermediate or the mutant is not directly mediated by furin-like PCs, but involves (also) other proteases. These results demonstrate that formation of the 180 kDa intermediate, consistently found only in rabbits, but not in man, is not essential for lactase transport: in all likelihood lack of selective pressure has led to species-specific processing of pro-LPH.

Processing of human intestinal prolactase to an intermediate form by furin or by a furin-like proprotein convertase

Human lactase-phlorizin hydrolase (human-LPH) is synthesized as a large precursor (prepro-LPH), then cleaved to a pro-LPH of 220 kDa which is further cut to a "mature-like LPH" of a size close to that of mature LPH, i.e. about 150 kDa (in the processing of rabbit pro-LPH the intermediate has a mass of approximately 180 kDa). By coexpression of human prepro-LPH with furin in COS-7 cells we show that furin generates a mature-like LPH. Radioactive amino acid sequence analysis reveals that furin recognizes the motif R-T-P-R832, a protein convertase consensus, to generate a NH2 terminus located 36 amino acids upstream of the NH2 terminal found in vivo at Ala869. This intermediate is ultimately cleaved to the mature LPH form by other proteases including the pancreatic ones. These data demonstrate that human pro-LPH, like the rabbit enzyme, is processed to the mature enzyme by furin or furin-like enzymes through at least an intermediate form that has, however, an apparent mass close to that of the mature enzyme.

Functional diversity and interactions between the repeat domains of rat intestinal lactase

Lactase-phlorizin hydrolase (LPH), a major digestive enzyme in the small intestine of newborns, is synthesized as a high-molecular-mass precursor comprising four tandemly repeated domains. Proteolytic cleavage of the precursor liberates the pro segment (LPHalpha) corresponding to domains I and II and devoid of known enzymic function. The mature enzyme (LPHbeta) comprises domains III and IV and is anchored in the brush border membrane via a C-terminal hydrophobic segment. To analyse the roles of the different domains of LPHalpha and LPHbeta, and the interactions between them, we have engineered a series of modified derivatives of the rat LPH precursor. These were expressed in cultured cells under the control of a cytomegalovirus promoter. The results show that recombinant LPHbeta harbouring both domains III and IV produces lactase activity. Neither domain III nor IV is alone sufficient to generate active enzyme, although the corresponding proteins are transport-competent. Tandem duplication of domains III or IV did not restore lactase activity, demonstrating the separate roles of both domains within LPHbeta. Further, the development of lactase activity did not require LPHalpha; however, LPHalpha potentiated the production of active LPHbeta but the individual LPHalpha subdomains I and II were unable to do so. Lactase activity and targeting required the C-terminal transmembrane anchor of LPH; this requirement was terminal transmembrane anchor or LPH; this requirement was not satisfied by the signal/anchor region of another digestive enzyme: sucrase-isomaltase. On the basis of this study we suggest that multiple levels of intramolecular interactions occur within the LPH precursor to produce the mature enzyme, and that the repeat domains of the precursor have distinct and specific functions in protein processing, substrate recognition and catalysis. We propose a functional model of LPHbeta in which substrate is channelled from an entry point located within domain II to the active site located in domain IV.

Furin, PC1/3, and/or PC6A process rabbit, but not human, pro-lactase-phlorizin hydrolase to the 180-kDa intermediate

Small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a large precursor (prepro-LPH) of 1926 amino acids. In the endoplasmic reticulum, prepro-LPH is split by signal protease. The resulting pro-LPH is cut to mature LPH directly (human) or via a 180-kDa intermediate (rabbit), most likely in the trans-Golgi network or in a later compartment. Antibodies directed against different regions of rabbit pro-LPH locate the cleavage site resulting in the 180-kDa intermediate between amino acid residues 79 and 286. This stretch contains the two sequences -Arg-Cys-Tyr-Arg114 approximately -Arg-Ala-Ser-Arg191 approximately, which are potential cleavage sites for subtilisin-like proprotein convertases. These sites are not conserved in human pro-LPH. By coexpression in COS 7 cells of rabbit prepro-LPH and proprotein convertases (PC 1/3, PC2, PC6A, PC6B, furin), we show that furin, PC 1/3, and PC6A generate a processing intermediate that is immunologically indistinguishable from the one observed in vivo. Furin, PC 1/3, and PC6A are all expressed in the small intestine as shown by a polymerase chain reaction-based approach and, more specifically, in enterocytes, as shown by in situ hybridization. These results suggest that furin, PC 1/3, and/or PC6A are responsible for the in vivo processing of rabbit pro-LPH to the 180-kDa intermediate.

Intestinal brush border glycohydrolases: structure, function, and development

The hydrolytic enzymes of the intestinal brush border membrane are essential for the degradation of nutrients to absorbable units. Particularly, the brush border glycohydrolases are responsible for the degradation of di- and oligosaccharides into monosaccharides, and are thus crucial for the energy-intake of humans and other mammals. This review will critically discuss all that is known in the literature about intestinal brush border glycohydrolases. First, we will assess the importance of these enzymes in degradation of dietary carbohydrates. Then, we will closely examine the relevant features of the intestinal epithelium which harbors these glycohydrolases. Each of the glycohydrolytic brush border enzymes will be reviewed with respect to structure, biosynthesis, substrate specificity, hydrolytic mechanism, gene regulation and developmental expression. Finally, intestinal disorders will be discussed that affect the expression of the brush border glycohydrolases. The clinical consequences of these enzyme deficiency disorders will be discussed. Concomitantly, these disorders may provide us with important details regarding the functions and gene expression of these enzymes under specific (pathogenic) circumstances.