Physiological roles of cell surface ecto-enzymes

维甲酸（RA）诱导HL-60细胞产生的外源型NAD酶活性是由CD 38引起的，CD 38的氨基酸序列与拟南芥ADP-核糖基环化酶同源。CD 38不仅催化NAD^+的水解，而且催化环ADP-核糖（cADPR）的形成和水解，后者是细胞内Ca ^2+库释放Ca^2+的新型介质或调节剂。本研究对CD 38及其相关胞外酶PC-1的功能和性质进行了研究。1. CD 38能够水解除NAD^+以外的许多化合物的N-糖苷键。2. CD 38大量存在于大鼠星形胶质细胞中，除了淋巴细胞，和细胞表面的CD 38被迅速灭活后，与酶底物孵育。3.可溶性和膜结合形式的PC-1，这已诱导在人Jurkat T细胞培养后与cAMP类似物，进行了鉴定和表征的酶和结构。4.两种PC-1分子均具有磷酸二酯酶/焦磷酸酶活性，可溶性PC-1的酶活性可用于寻找与其相互作用的分子。5.细胞外基质中的糖胺聚糖如肝素和硫酸乙酰肝素能够与PC-1结合，并以与酶底物竞争的方式抑制其磷酸二酯酶活性。6.人PC-1的同源物也存在于秀丽隐杆线虫中，并且它具有磷酸二酯酶/焦磷酸酶活性。酶活性定位于细胞表面。因此，PC-1可能作为粘附分子与细胞外基质中的糖胺聚糖结合。

Ecto-form NADase activity induced by retinoic acid (RA) in human HL-60 cells is due to CD38, which has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase. CD38 catalyzes not only the hydrolysis of NAD^+, but also the formation and hydrolysis of cyclic ADP-ribose (cADPR), that is a novel mediator or modulator of Ca^<2+> release from intracellular Ca^<2+> stores. In the present study, we investigated the functions and properties of CD38 and its related ecto-enzyme, PC-1. 1. CD38 was capable of hydrolyzing the N-glycoside bond of many compounds other than NAD^+. 2. CD38 was abundantly present in rat astrocytes in addition to lymphocytes, and the cell-surface CD38 was rapidly inactivated upon incubation with the enzyme substrates. 3. Soluble and membrane-bound forms of PC-1, which had been induced in human Jurkat T cells upon culture with a cAMP analog, were identified and characterized enzymatically and structurally. 4. Both PC-1 molecules possessed phosphodiesterase/pyrophosphatase activity, and the enzymic activity of soluble PC-1 could be utilized to search for its interacting molecules. 5. Glycosaminoglycans, such as heparin and heparan sulfate in extracellular matrix, were capable of binding to PC-1 and inhibited its phosphodiesterase activity in a manner competing with the enzyme substrates. 6. A homologue of human PC-1 was also present in C.elegans, and it possessed phosphodiesterase/pyrophosphatase activity. The enzymic activity was localized in the cell surface. Thus, PC-1 may function as an adhesion molecule to associate with glycosaminoglycans in extracellular matrix.