Regulation of urease levels in jackbean cotyledons / by James Arthur Cook.
| Author/creator | Cook, James Arthur author. |
| Other author | Sehgal, Prem P., degree supervisor. |
| Other author | East Carolina University. Department of Biology. |
| Format | Theses and dissertations |
| Production | 1969. |
| Description | 57 leaves : illustrations ; 28 cm |
| Supplemental Content | Access via ScholarShip |
| Subjects |
| Summary | Factors controlling urease levels in sterile cultures of jackbean cotyledons were investigated. Urease levels were determined over a period of time by successive measurements of urease activity from extracts of the cotyledons. Urease activity V̕as measured in micro-Conway dishes by reacting aliquot portions of the enzyme solutions with an excess of standard urea solution. Units of urease activity refer to the ml of HCl equivalent to the ammonia that would be evolved in the volume of extract per gram fresh weight of the cotyledon. Cotyledons in cultures behaved the same as cotyledons harvested from seedlings at different ages. In 4 to 5 weeks, the levels of urease fell to 1% of the initial units in soaked seeds. High temperatures, lov̕ pH, and the presence of ethylenediamlnetetraacetic acid, hydroxyurea, or actinomycin D in Miller's medium led to a decrease in urease units. Urea, thiourea, and increased growth favored higher urease levels in cultures of jackbean cotyledons. It is known that ammonium ions inhibit urease activity and possibly repress it. A continuous flow of sterile medium with and without ureas was maintained to avoid excessive accumulation of ammonia. In the control, compared to the urea-treated set, urease levels fell. A 60-70% increase in the urease levels in the urea-treated set was observed within 12 days. Thus, an increase in urease in relation to the control was attributed to substrate protection. Urea has also been shovm to Induce urease formation in higher plants. In experiments with thiourea added to the culture medium, an equivalent amount of nitrogen as nitrate was substituted for ammonium salts. An increase in urease levels ranging from 155 - 199% was noted in thiourea-treated cotyledons. Thiourea inhibits nitrate reductase. Lower urease levels in the set with no thiourea may have been due to inhibition by metabolically produced ammonia. Induction of urease by thiourea has been shown in bacteria. Extracts of cotyledons in cultures with low thiourea concentrations contained 30% more ammonia than ones in high concentrations of thiourea within 5 days. In experiments with cell-free extracts, there was no indication of protective action of thiourea on urease against proteolytic activity. When cotyledons are grown on agar with thiourea, the metabolic utilization of ammonia in protein synthesis, lower protease activity and possible Induction of urease may sustain higher urease levels. A central role for ammonia as a regulator for changes in urease levels is hypothesized. Such a hypothesis explains earlier work on the phylogenetic and ontogenetic occurrence of urease as well as data presented here. Amides or compounds of the ornithine-urea cycle are abundantly present in urease rich seeds such as soybeans, jackbeans, and watermelon. Maximum synthesis of urease in cotyledons of ripening seeds may be stimulated through the incorporation of free ammonia into these compounds. Conversely, in cotyledons of germinating seeds reactions favoring breakdown and/or utilization of amides and other nitrogen reserves are predominant. Proteolytic activity rather than synthesis dominates the metabolism of cotyledons in germinating seedlings. Under these conditions an excess of ammonium ions may inhibit urease or repress the enzyme synthesis. |
| General note | Submitted to the faculty of the Department of Biology. |
| General note | Advisor: Prem P. Sehgal |
| Dissertation note | M.A. East Carolina University 1969 |
| Bibliography note | Includes bibliographical references (leaves 45-47). |
| Genre/form | dissertations. |
| Genre/form | Academic theses. |
| Genre/form | Academic theses. |
| Genre/form | Thèses et écrits académiques. |