Abstract: An amplifiable ?-lactamase substrate is provided comprising an enzyme donor fragment of ?-galactosidase linked to form a ring to a ?-lactam ring that is a substrate for lactamase and upon opening of the ?-lactam ring the enzyme donor fragment becomes linearized. The cyclic substrate only weakly binds to the enzyme acceptor fragment of ?-galactosidase. The substrate finds application for the sensitive detection of ?-lactamase for direct detection of the enzyme or when the enzyme is used as a label.

Abstract: An amplifiable ?-lactamase substrate is provided comprising an enzyme donor fragment of ?-galactosidase linked to form a ring to a ?-lactam ring that is a substrate for lactamase and upon opening of the ?-lactam ring the enzyme donor fragment becomes linearized. The cyclic substrate only weakly binds to the enzyme acceptor fragment of ?-galactosidase. The substrate finds application for the sensitive detection of ?-lactamase for direct detection of the enzyme or when the enzyme is used as a label.

Abstract: Short enzyme donor fragments of ?-galactosidase are provided of not more than 40 amino acids, where the short fragments are used as a label and may be substituted with a wide variety of organic compounds, particularly polypeptides having independent functional activity. The enzyme donor finds use in competitive and non-competitive assays, monitoring intracellular events, or other processes where a sensitive non-interfering label is desired.

Abstract: Protein binding assays are provided for determining IP3 in a sample employing as reagents a conjugate of IP3 joined at the 2-oxy through a bond or linking group to a detectable label and a truncated portion of the extracellular fragment of an IP3R. The reagents are combined with the sample and the amount of IP3 determined by means of the detectable label. The conjugate with the enzyme donor fragment of &bgr;-galactosidase or a fluorescer is specifically described.

Abstract: Improved results are obtained using in an enzyme fragment complementation assay, as an enzyme donor a fragment of &bgr;-galactosidase of from 36 to 50 amino acids joined to a ligand capable of binding to a receptor other than an immunoglobulin or fragment thereof. Conveniently, for enzyme assays, enzyme binding site inhibitors are conjugated to the enzyme donor, whereby binding of the enzyme to the binding site inhibitor results in a reduction in the turnover rate of &bgr;-galactosidase in the presence of EA and a substrate capable of producing a detectable signal. Analogously, ligands for membrane receptors may be conjugated to the ED for measurement of membrane receptors.

Abstract: Short enzyme donor fragments of &bgr;-galactosidase are provided of not more than 40 amino acids, where the short fragments are used as a label and may be substituted with a wide variety of organic compounds, particularly polypeptides having independent functional activity. The enzyme donor finds use in competitive and non-competitive assays, monitoring intracellular events, or other processes where a sensitive non-interfering label is desired.