Substance P Radioimmunoassay Using Nα-Tyrosyl-substance P and Demonstration of the Presence of Substance P-like Immunoreactivities in Human Blood and Porcine Tissue Extracts

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Sensitive and specific radioimmunoassay for substance P was developed using synthetic substance P and 125I-Nα-tyrosyl-substance P. Substance P-human α-globulin conjugate was used for production of anti-substance P antisera in rabbits. Synthetic substance P was used as a standard and the dextran-coated charcoal method was employed to separate the free peptide from that bound to antibodies. No crossreactions by physalaemin and eledoisin observed in this system proved its high specificity to substance P. Nx-Tyrosyl-substance P and [Tyr1]-substance P showed the displacement curves indistinguishable from that of the standard substance P. Neither substance P5-11 nor substance P6-11 competed with the tracer at the concentration used. The minimum measurable dose of substance P by the assay system was 2.5-5 pg/incubate. Utilizing the system, human plasma samples from 42 healthy volunteers of both sexes were shown to contain immunoreactive substance P in amounts that averaged 298pg/ml in male and 251pg/ml in female. Substance P-like immunoreactivity was also demonstrated in hot-water extracts of porcine duodenum, jejunum, ileum, cecum, middle colon, rectum, pancreas, stomach and pituitary. The highest concentration (379 ng/g wet weight of organ) was found in the pituitary, and the ileum (7.9ng/g wet weight of organ) and jejunum (1.9ng/g wet weight of organ) were rich in the contents.
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